I believe Mike first mentions creeples here.

↓ Transcript
Panel 1:
Mike: (To Lisa) How about this. Every week I'll spend some time on my board game thing, and you spend time on WoW.
Panel 2:
Mike: I'll even play WoW with you sometimes if you help me make creeples once in a while. And then we spend one night a week doing something completely new, together.
Panel 3:
Lisa: (winking nose) Like rock-climbing?
Mike: Does that interest you?
Lisa: No.
Panel 4:
Mike: Oh, what would you like to try?
Panel 5:
Lisa: I've never played Rift..
Mike: (walking away) Rock-climbing it is!