If someone is fast enough to catch a group I wouldn't find it offensive to ask "may I sit in for a bit". Just be observant. If you see they're doing some kind of workout rotation, don't join and screw that up. If they're just cruising at tempo with some pretty long and easy pulls on the front, whatever, won't affect anything.
Not being mean about it, just pointing out a few obvious issues I could see coming up:
-The group has a turn in the route that doesn't match your route while you're NOT on the very back. Thus creating a problem. Or, if you choose to sit on the very back, you'll have to be a really good back door rider to let people in without being an annoyance.
-The group is doing some kind of special rotation you're not used to, but they are.
-You're on a flat bar bike and they're not and they take the t-junction intersection at full bore road bike speed and you can't maneuver as well.
Just things to consider. But again, no harm in asking.
I've also caught smaller groups OR larger B or A minus groups and had the issue of them sucking wheel until a hill. So, now I will sit back about 100 yards and wait for a hill and then surge past. I find it infinitely annoying when a 2-wide line takes up the entire lane and won't squeeze in for me to pass. Making me pretty much pass over the centerline. THAT is ******y. Hold the rotation for a second and tighten the F up.