Don't know about the changing groups thing. Use your own judgement. Are they the sort who will stop if they notice you are missing and will wait. Probably a bit poor if they're hanging around thinking you have a puncture when actually you've buggered off with someone else. Like wise, if they are the sort who just get on with it and don't give a stuff if you get dropped etc then why not.
The sprinting thing makes me laugh. Generally sprints are for boundary signs, eg entering a town but if they are the sort which don't bother then you just look odd. Sounds like you just went off on one by yourself anyway, which is funny.
Most groups I've riden with, it's usually the younger members who sprint for signs and the older (i.e. anyone over 21) will say leave it for a race and see it as a bit childish. However, having moaned about how childish it is for 100miles, will then sprint for the second from last boundary, winning it with fresher legs from having stayed in zone 2 all ride, and thus proving that they could have won all the others if they'd bothered. They'll leave the final boundary for the youngsters again and this is the only boundary that really matters and it's not worth killing yourself for a club ride. Unless you are on holiday, in which case the whole of life becomes a race.