Depends on the group. I know most of the folks who train on the route near my neighborhood. If I see the blue Elevate/KHS jerseys, I know I can't hang with them long, even drafting. It's a pro team and even their "slow" rides would grind me up into paste (their weekly "recovery" ride is faster than I can manage on my best day at full gas). And I'd interfere with their paceline. At that level they know each other intuitively. A new person would probably mess up their training. Nice enough fellows, though. We wave in passing.

The club I hang out with has A and B group rides. The B group is very informal and welcomes anyone who wants to ride along. They only ask for the usual stuff -- competence, be careful, be respectful of other folks who use the MUP since we sometimes need to use the MUP to get from one public road to another, hold a line, call out significant hazards, etc. It's no-drop and there's rarely more than a few hundred yards separating the front and back of the group.

Pretty much the same with the A group. I haven't recovered my strength enough to hang with them for long, but they welcome new folks or occasional participants who aren't official members. Same basic policies -- ride safely. That group usually gets strung out over the miles so they have a few regroup points depending on the route and distance. But what usually happens on their longer rides is the group begins with 20-30 folks and as they get strung out over distance they'll form two or three ad hoc smaller groups to tow each other along. Works out fine.