I didn’t check out very much of the East side of the island so I can’t speak to it.
Kamala is better, though can still be quite busy as all of the Beaches near Patong can be. The good thing is that it is still close enough to Patong that if you want to go there for an activity, shopping or dinner its an easy trip (except during rush hour).
What are you aiming for? What kind of budget, what kind of activities? Also what time of year? From what I understood the on season vs. off season are quite different so that could be a huge opinion shifter on where would be good.
The further north you go the quieter it gets. The northern end of Bang Tao is alot quieter but harder to get to and tends to be more high end. The very northern part up near Nikki Beach club can be a great quietier spot good for a larger group. Be warned that “commercial activities” are theoretically banned in the entire Cherang Talay area so if you do work with any sort of a local planner, they may be bribing (or taking a risk on a fine) their way to organizing anything. The southern end of Bang Tao may have a pollution/sewage problem. The river at the very bottom did not smell or look pleasant at all and it was flowing right into the ocean.
Surin Beach is nice, but small and still was quite busy. It also had pollution problems (much of Phuket has open sewage and it grew far too fast for the infrastructure to keep up so you could encounter pollution/sewage at many of the beaches). They were busy working on a treatment plant back when I was there, it was due to be finished months ago but hadn’t been completed when I was there.
If you want to be closer to Patong (it is where all the action is on the island and the majority of nightlife/restaurants are there) I liked the look of Karon and Kata but I only really drove through there and stopped briefly to have a look. I stopped at Kata Noi for a few hours, very nice but very small.
Chalong/Rawai beaches seemed rather narrow and often packed with boats but could be reasonable.
Island wise, a friend of mine who went to Ko Phi Phi a few months before warned that its a huge tourist trap, filled with trash and not a very pleasant experience. I skipped it but my wife and her visiting friends went and they had much the same feedback.
Photographers I worked with spoke very highly of Ko Racha but I didn’t get the chance to go see it as we ran out of time. It sounded like a great spot and far less well known and touristy than Phi Phi. The way they described it, there are two sides and people always flock to one or the other depending on the season so if you cross to the opposite side where the people are it’d be a bit quieter.
For 100 people you’ll have alot of logistics to figure out, unfortunately I can’t help there.