In the build I played for preview I experienced 2 guest character join-ups (both Cor) and both lasted for less than 10 minutes, so... there's a big bloody difference between this and an actual party member, female or otherwise. Guest characters don't have skill trees or any of that stuff either. They have a few team-up attacks, but those are context sensitive. They're NPCs in the truest sense, where you can at least exert some control over your party members.
If we want to get into semantics: they're not party members, they're guest NPCs.
I'm not saying they'll all be for 10 minutes, though my general understanding is that guest characters won't be with you for extended period of time. Think Beatrix or Blank joining you in FF9, or Sephiroth in FF7's flashback as opposed to something like Edward or Yang in FF4. This is one of these things where, ha, I'm trying to temper expectations a bit, as people are only going to be upset when guest stuff isn't as extensive as they'd hoped, even though the game is fine without it.