Do either or both of your main characters have the Lone Wolf talent? For each that do, it will reduce the maximum party size by one (max 4). In that case, Jahan should say something about your party being full.
If you have an AI personality set for your non-lead main character, they may object to hiring someone during the conversation. Either select them as leader to do the hiring, or leave one main character downstairs, and just hire Jahan with the other.
