I always send them an awkward tell pretending like their tell interrupt some conversation I was having with someone else. A lot of times I get "...." or "wtf man?"
Ex:
Vivain: HI, want to join <Fluffy cats hero>? We are level 25 and have perks!
Me: Have you tried <insert gross sounding, fake porn site here>? It's got the best camera angles...
Vivain: What?
Me: Uh..........nvm. Sure, I'll join your guild... it sounds perfect.
Vivain:..........
Vivain: nvm