I think it can cater to more than one person at the same time, just my opinion though...

^same

I believe it can only be one room at the same time, but that specific room caters to all of its users, as long their wishes don't require the entire room to change.

^what lector said. For example, in Deathly Hallows, the Trio needed the room where everything is hidden. The last people were gone, Augusta was going to find Neville, and Ginny was waiting like her family told her to. Harry let her go into the war because he needed everyone gone for it to shift back

Nope. The original users gotta get out