Hm... Some enemy NPC's. I recommend you guys take the top of a Braniac bottle in Gotham or somewhere with minimal crowd. I bet that lower levels will be near your hangout if you use the Funhouse location in Gotham. If you want to make it super secret then I suggest moving to a low to no populated area (in terms of NPC's and players) with enough area/volume/space. This would allow you to make it exclusive, secret, and perhaps fun.
Honestly, This is what I am most looking forward to. I can't wait to finally play the game and maybe even run some instances with all the people here in the crazy... I mean Funhouse.