The International Brigades were, from the outset, a Comintern project. The Soviet Union controlled the Comintern, and thus exerted significant strategic and ideological influence over the Brigades. But the primary organising and recruiting bodies for the Brigades were national communist parties and other left-wing organisations (like trades unions).

There was a substantial propaganda and proselytisation programme in working class communities around Europe and North America, with communist recruiters trying to drum up volunteers to travel to Spain to fight fascism.

It's not a conventional scholarly source, but the opening scenes of Ken Loach's film Land and Freedom (one of the best English-language films about the civil war) are a pretty accurate portrayal of how that recruitment process often went: a communist ideologue addressing workers in a working men's club or union hall, exhorting them to join the resistance to Franco.

(Note that Land and Freedom — like Orwell's Homage to Catalonia, also a fine account of foreign fighters in Spain — focuses on the anti-Stalinist POUM militia, not the International Brigades. But the recruitment scene is nonetheless illustrative.)

The really challenging part was getting volunteers to Spain: the Comintern established a depot at Albacete, which acted as the logistical and training headquarters for the Brigades, but getting there was no mean feat. Non-interventionist governments in Britain and France both actively discouraged or forbade their citizens from fighting in Spain, and the French authorities frequently attempted to prevent people from crossing the border — forcing them instead to sneak across the Pyrenees or travel by sea from Marseilles.

Fortunately for the Brigaders, though, that border is long and porous, and attempts by Britain and France to prevent travel to Spain didn't stop more than 30,000 foreign nationals from fighting there.

anybody other than the Germans help Franco?