Wide, low pressure tires will make it harder to keep up with the group. If you haven't ridden with a group before, or are not sure that you can keep up with a group, try one of the rides listed as Easy or New Member pace. No one gets dropped on rides advertised like that. Average speed is 10 mph or less while moving. Non-challenging terrain, just fun.
Group moves at the pace of the slowest rider.
C For average cyclists. Average speed is 9-12 mph while moving. Flat to rolling terrain. Group remains intact. B For good, competent cyclists.