Personally, if I were in a group and a rider comes up, I would not like them following close at all. I would say 4 or 5 seconds back. This is taking into consideration that they are sportbikes and are riding at a brisk clip. This really does not apply to cruisers or tourers. Follow them until they stop (sportbikers always do to brag...he-he...) and pull up and talk to them. If they ask you riding questions, anwser honestly, they are sizing you up. Now, you just made some riding buddies. What we normally do is have the new guy ride 2nd to last with a senior guy behing to check you out and make sure you are safe. If you are going to go fast around curves, you want to trust the guys around you.