It all depends on the group. It is important to do your homework on the group and to find a group whose goals and expectations are similar to yours. I find it the most enjoyable to ride with others who cause me to push myself but don't make me feel out of place.

Group rides do increase the chances of collisions with other riders but selecting a riding group of competent and safety minded individuals reduces that risk. Groups of big egos and show-offs increases the risk. Riding with newbies (we all were one at one time) is also risky but you can mitigate that risk by keeping their newbie status in mind and adjusting your riding style accordingly. Your ride leader sets the tone for the entire ride, so make sure you have a leader that you trust.