I never participate in "group tips" because you never know what happens to the money. I know of a dive shop owner who collected a group tip at the end of the week and then told another employee at the shop the guests gave enough so he didn't need to. On all the trips I put together, I never collect a group tip. I will try and find out what is the norm and if anyone asks I just tell them what I found the usual or customary tip to be.
Interesting. On several occasions (non-diving trips, safaris) I've been asked by the other clients to collect the tip since "as the American, you're used to that sort of thing." I usually do as you do, find out what the norm is and let people know if asked. I'm usually up front about how much I'm tipping and why.