Partner cards, why are they greyed out half the time?

[Scyris]

I am wondering why partner cards seem to be greyed out half the time for me, it won't let me invite them to my party. Any idea how I can fix this or does it just happen randomly?

[JeyKama]

There can only be one NPC of each type in each multiparty room, so if other people in the room are using NPCs, you can't use the same ones.

[Scyris]

Ahh that explains it. Thanks.