I bought this game used yesterday and my wife and I played it for around six hours. We didn't unlock anybody (other than Bravo team in the first mission), but after some research I've found that you need to go through some sidequests at different headquarters to get the locked characters.
We are now at Nick Fury's hideout on the pro-registration side (after Chemical Plant). Nick Fury asked to go recruit Storm and Luke Cage (I think). Anyway, is it too late to unlock some characters since we didn't do it at the past headquarters? Would be a bummer to have bypassed chances to get some people.