Since a lot of old fans Will probably want to reserve a spot for Minsc, that means you can take 2 of current NPCs out of Act1 - now choose which 2 are you taking?
It's rumored, and possibly confirmed by peeking through game files, that there will be four more companions to choose from. A total of eight before the Act 1 lock.
Assuming Minsc is one of them, there will be three more we haven't seen. Or maybe we've already seen them as NPCs in Act 1, like the ranger Karlach and druid Halsin.
Are they confirmed before the end of Act I, or just confirmed? There's a difference, a world of difference.
The files for them are similar to the other origins characters, so before the end of Act 1. At least, the 3 people usually mentioned are apparently origins. Except, I'm pretty sure one of them isn't an origin, so what the files looks like might mean nothing...
As for getting companions post Act 1, I doubt it based on their technical design reason (not story reason) for the locking the party to start with. We will get temporary companions for certain quest I'm sure, we already have a few in the EA.