After you unlock your first two allies (yes unfortunately I had to reset your current unlocks) you'll see an 'Ally Inventory' button in the menu.

From that point onwards, everytime you encounter an ally (and die or win the run) you will collect it in your inventory, choose two to be active.

Unique ally abilities based on tags/developers/genres (similar to enemies) is up next!

I'll move this build to demo tomorrow morning, if all goes well.