I think if you worked with others, 5:30 is a pretty ok time for somebody new, but it doesn't really tells us much. Drafting helps immensely, shaving off more than an hour in your case I'm sure. When I'm out alone I'll be taking 230watts to go 22mph, but in a group it's something near 150watts, which is basically me not even pedaling. I specifically try to avoid getting in big groups on rides I paid for, because I'm in it for the personal challenge, not to have others do 80% of the work for me. I may trade a few pulls with a random person I encounter, but we part ways pretty quickly. Other people sign up for these things because they really enjoy group setting and the scenery, and other groups are out there to work together and blaze a sub 4 hour group century. Doing one in a group feels like a cheapened challenge to me - I'm pretty antisocial.

As for stops, if you care about your time, fill your bottles and grab a piece of food to eat on the bike. You don't really need much food for a century, two snacks is probably enough if you've also got calories in your bottles.