If pace / drop tolerance wasn't posted prior to the ride, the default assumption should be no drop. However, this is complicated. I've been riding with the people I usually ride with for years and know their strengths and weaknesses well. I generally assess what I know of the people who are going to show up for my group rides. I know if certain people show up, it's going to be fast. If other people accept, I know it might be a bit more leisurely. It's incumbent on every rider to honestly assess their fitness vs. others in the group and proceed accordingly. If you don't know the capabilities of other riders, I'd say you have to be willing to deal with the consequences. If it's a strong group with one weaker rider, I do think it's a bit unfair for the pace of the entire group to be dictated by the slowest rider. There have been rides for me where I'm getting my ass kicked and I have no problem tell the others to just go. I personally don't feel comfortable making people wait for me. This does not seem to be the case in this ride, given that others started to blow up way too early. I'd say it sounds like a possibly inexperienced group.