I think I have been working on another similar question of yours for a while now. It's not done yet (sorry). I feel I can at least highlight a few points of relevance here. Feel free to ask additional questions, as the longer answer will take some time.

The first thing to keep in mind is that Mussolini left the Italian Socialist Party during a period going from October 18^th (day of publication of the famous opinion piece on the "active and operating neutrality") to November 29^th 1914 (when the Party Direction ratified the expulsion motion approved by the Socialist Section of Milan on the 24^th of November), two weeks after the first issue of his Popolo d'Italia came out advocating for the Italian intervention. At this point, Mussolini - for a socialist, or perhaps better from the perspective of the Italian Socialist Party - was no more of a socialist than the Kaiser. This didn't necessarily reflect Mussolini's own conceit, or that of those who believed his treatment to have been unfair, but for those who agreed with the position of the Party - and in no little part due to how acrimonious and violent the departure was, and perhaps more so due to Mussolini's brazen act of defiance with the opening of a second "socialist newspaper" in Milan - Mussolini was gone for good.

Mussolini then founded the Fasci di Combattimento on March 23^rd 1919. An extremely small organization (so small that Mussolini himself appeared far from certain of its potential and practical value) catering chiefly to "combatants" and pairing ideas of left-wing radicalism with the new popular themes of "trencherism" and productivism.

As you can see, there's four and a half years between the two moments, and far from uneventful years. The entire path through the war wasn't merely an interlude, but a defining moment of that "combatant" identity that would contribute significantly to the "early days fascism" of San Sepolcro; the Bolshevik revolution would alter the landscape of international socialism and the historical perspective of the socialist movement itself; the crisis of Caporetto changed the Italian political landscape (somewhat) and especially gave impulse to new forms and modes of "war discourse" (the comparatively more "modern" ones, one might be familiar with from the work of Mosse); the industrial expansion and disruption of the Italian economy during the war, the severe repercussions on international trade, with their corollary of supply shortages and financial struggles that extended the impact of wartime economy into peacetime by several months; all these things going on mean that we should not think of the passage from "interventionism" to "fascism" as one single leap.

It was - to steal my own image - more of a "wandering through the desert", until both sides - socialists and fascists to be - found themselves in a new land.

In the beginning, the issue was "interventionism"; or - for the socialists - the attitude that the Italian Socialist Party should take with regards to the clash between the Central Empires and the Triple Entente. By and large, the Italian socialists (without capital s) favored the Entente. By and large, the Italian masses wanted to keep out of the war.

Not wanting to die probably doesn't need an explanation. The sympathy for the Entente was driven by a series of factors: the wide impression created by the "rape of Belgium", that consolidated the image of the German "horde" assaulting peaceful Countries; the republican and more "progressive" reputation of France; the identification of the Central Empires with the Triple Alliance, that the Italian "subversives" had fought against tooth and nails for over three decades as the diplomatic incarnation of the authoritarian and reactionary policies of the Italian State.

As such it was almost natural for the Italian Socialist Party to assume a thoroughly "neutralist" stance, but focusing almost exclusively on the threat of a war on the Austro-German side. The result was an "absolute" neutrality that the vast majority of the Party leaders and militants filtered through various degrees of "Francophilie".

When Mussolini pushed for a change of direction, then, it was to open the way to an action of the Socialist Party favorable to an intervention on the side of the Entente. It is not exactly clear if Mussolini believed he could actually lead the Party to follow this new direction. There were, as noted, various voices sympathetic to the Entente, but very few went so far as to promote the intervention. He might have been overestimating the degree of openness to this transition within the militant base or his ability to drive such transition; but more probably he felt dissatisfied with how the Party Direction had been running things (at least) since late July.

In any case, the Party Direction rejected Mussolini's attempt; and, after a brief attempt to persuade him to "take a leave of absence for health reasons", promoted and sanctioned his expulsion. The Party (the militant masses, through the Sections) followed the Direction almost unanimously in the condemnation of Mussolini. On this point – to be fair – if Mussolini had failed to turn the Milanese Section, there was very little hope for other socialist interventionists; so that those who wished to support the Italian intervention would rather just leave or wait for an executive expulsion to be ratified once the Section had acknowledged their “anti-socialist” action.

There were a few passably recognizable figures that joined Mussolini – or better, joined the interventionist front before or after Mussolini – but I can hardly think of one who actually “followed” Mussolini as it wasn't really clear where he was going (as of November 1914, Mussolini had a newspaper with a diminutive redaction room that he was running with funds putting together with the objectionable assistance of Filippo Naldi). The largest amount of support for Mussolini among socialists came from those “socialists” outside of the Party: Salvemini would certainly be the most distinguished name (if only for his twenty years of anti-Fascist militancy), Lombardo-Radice who had left the Party in September and reportedly played a role in Mussolini's “conversion” with their letters exchange. Massimo Rocca would be a recognizable name, but only because the odd-anarchist had been the one “calling out” Mussolini's phony neutrality under his pseudonym of Libero Tancredi. A couple of members of the Avanti!'s redaction would join Mussolini at the Popolo d'Italia, albeit not immediately (that I know of) due to contractual obligations.

Other Socialists would cross more or less the same bridge, but without necessarily drawing direct impulse from Mussolini's initiative. Lido Caiani – the chief editor of the official newspaper of the Socialist Youth Federation and member of the Central Direction – had come under attack for his “Francophile and warmongering attitude” already by late September, and was arguably on his way out from the Avanguardia and from the Party, before Mussolini manifested any uncertainty over the intervention.

Still, outside the Party, there were the Syndicalists, split between the influential leaders De Ambris and Corridoni who had come out as interventionists quite early in the conflict, and the neutralist Armando Borghi who had retained control of the vast majority of the Sections (albeit losing those of Milan and Parma).

Inside of the Party, apart from the exceptional case of Cesare Battisti – obviously an interventionist, but the Party would not dare invoke disciplinary sanctions at the time – there were figures who rejected the Party's official line, but didn't wish to cross the interventionist bridge. Alceste Della Seta – who's best known for his exchange with German socialist Sudekum during the Direction meeting of late September – had repeatedly protested the position of the Direction and eventually left the Party, but only in 1915, in order for his move not to appear supportive of an interventionist platform, and essentially retired from active politics.

Among the interventionists, there were the right-wing reformers of Genoa: a strong bunch quite influential within the Chamber of Labor, tied to the commercial tradition of the city; and a significant fraction of the Roman Section (that in fact lost more than a few members along the way) where a strong republican and “heterodox” subversive tradition existed (consider how some of the anarchist leaders of the 1919 Arditi del Popolo had been war volunteers).