Can you clarify what you mean by "believe in"?

If you take it as a given that the reconstructed language (for which the group — as speakers of said language — as here named) is actually something that was spoken, then there were speakers, and the Proto-Indo-Europeans were simply whatever group spoke it, and they may very well have another name to historians in addition to that (e.g. the Jurchens could be called "Proto-Manchus", though they typically aren't).

There's some disagreement among historical linguists about how we should treat reconstructions. Some will say they are simply a set of correspondences unrelated to time (and therefore not necessarily all in existence in a single speech variety at a single time), while others take the view that a reconstruction is an approximation of an actual speech variety in an actual point in time in the past.

In East Asian linguistics the former view tends to be a bit more typical (in my experience), but from what I've heard from Indoeuropeanists there are a number who subscribe to the latter view. What position you take on that issue necessarily affects how you'd view the possible existence of a single group of speakers for a reconstructed form.

To answer your question is therefore a little more complicated than just a yes or no. Serious historical linguists both will and won't depending on other theoretical considerations, and since linguistic reconstruction isn't always a perfect parallel for archaeological viewpoints and even less often a perfect parallel for genetic lines, the answer becomes even more complicated.

I don't know whether "serious historians" believe in them, but if you don't accept as fact that there was some group of people speaking some (varieties of a) language ancestral to the modern Indo-European languages, then you're basically rejecting linguistics as a discipline.

It is painfully clear that IE languages descend from a single ancestor. The sheer number of cognate lexical items and grammatical structures that we've reconstructed admit no explanation other than common descent. While it's conceivable, as /u/keyilan points out, that our reconstructions incorporate features that never coexisted in a single speech community, that's a problem with our reconstruction. It's certain that no one ever spoke a language identical to our reconstruction, at any rate: reconstructions can't be complete. We'll never know the range of sociolinguistic variation in vowel space for PIE, for instance, and we'll probably never know the exact nature of the PIE accent system.

Much like it's essentially impossible to perfectly reconstruct the common ancestor of any two given species, it's impossible to perfectly reconstruct the common ancestor of two given (obviously related) languages. None of this changes the fact that there was some common ancestor for those two given species, and at some point there was a common ancestor for modern IE languages.