A good idea I think would be to have some sort of status option so that other riders can see what sort of ride or session you are doing and decide to join or not.
For example if I’m doing intervals and someone rides along with me when I’m doing a nice steady pace I always feel a bit bad when I suddenly start pushing out power, as if I’m telling them I don’t want them with me.
Or when I join and am doing a steady hour or two it would be great to know there are a few other people doing a steady 200-250 watts or whatever, and I might join in near them or catch their wheel.
It’s very similar to what people tend to do in real-life groups - deciding to ride with other people depending on what session they are doing, or do your own thing.
It could be an option on log in to set a status or session description, and maybe a small icon next to the rider’s name so you know they have declared their intentions and can click on it if you like.