It was not in the EA, so no one has a definitive answer, that I have seen. In one of the interviews or from the hands on press junket, someone said that you can hire 1 merc per class, or some such, that you customize (maybe that means you get as many mercs as there are classes, but it's not unlimited mercs). I would assume you get access as soon as you can access your camp, which is after the intro/tutorial.
Quick edit: I just remembered and scanned through a
Wolfheart video, and it shows him using a mostly custom party in the area right after the tutorial:
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