I was torn on this at first, but I think I agree with sfa. So many people panic unexpectedly during the swim start that I think if it really is a race simulation the mass-ish start is something that could be invaluable. Your first time, transitions will be relatively slow compared to your second, third, etc, anyway, and you can totally practice those at home (once a week - or more often - when you get out of the pool/practice swim, practice transitioning to your bike, same for bike/run, etc).
I did not expect to panic the first time I got in the water with other people. I was a strong swimmer and had pool swam with other people plenty of times and practice open water swam, but I got completely overwhelmed once people started moving. I was able to get in and breastroke the entire thing (which is more than I can say for a lot of people their first time), but I was way more panicked than I expected.
The only way to get used to open water swimming with other people is... open water swimming with other people.If you're good with race anxiety and comfortable with open water, this would erase one other big variable. I think there's less to mess up in a transition (unless you are trying to win) than there is the risk of freak-out-on-GO (I wonder if there's an official term for that kind of performance anxiety).