Kinda depends what you want out of the day. It's between you and your mates. There's no set "etiquette".
I've done events with faster friends and been entirely happy to let them zoom off at their own pace.
I've done events (sometimes the same event) when I've hung around to accompany/encourage a slower friend up a big hill, only for them to promptly disappear into the distance on the downhill, which is fine. I soon caught up again when the road went upward.
It would be quite different, I think, if it was just a group ride on the road, rather than an event with directions and other riders and stuff. In that case, just dropping someone and pressing on is pretty rude, IMO, unless it's been discussed beforehand. I've had that done to me and have been somewhat pissed off.