I've cooled down (again) and decided that I'm best off discussing this with the club president:

Hi <name>,

I'd like to let you know about something that happened on Saturday's group ride. About 8 of us met at Centennial to ride a loop of the Half Full bike route. Before the ride we talked about trying to maintain a 15-17 mph average. As it turned out, I was having a terrible ride and fell off the group before we'd even gone a few miles. I wouldn't have wanted to hold the group back, but nobody even realized they had lost me until very much later so nobody came around to check on me. I'm an experience rider, familiar with most of the roads we were riding, and well capable of taking care of myself on a solo ride so I was never in any danger. However, what if I had been an inexperienced rider unprepared to take care of myself, thinking I was on a group ride? We had never discussed if this was a no-drop ride or anything and I don't think anyone was really leading the ride. Still, from my view there was a real failure of responsibility. From a <Tri Club> perspective, I think there needs to be some sort of expectations for group rides to protect the <Tri Club>. On every ride organized "publicly" through the group email list somebody needs to be in charge and everyone needs to know the rules for the ride before wheels start rolling. I feel like I'm tattling to the teacher, but I thought that you should be aware of this for the good of the <Tri Club>.

Thanks, Nicole