Is that a 15 mph
pace or 15 mph
average speed?
At this time of year, riders are getting faster. So a 15 mph average ride might be 15.5 or 16 in reality. With stops and hills, that could be 17-18 on the flat sections. But some groups really do ride at their advertised speeds. It's always good to email or contact the ride leader if you are a new rider.
Like johnny99 said, a 15 mph on the flats ride probably won't ride too close together. At 17-18 mph, it would have more drafting.
Here's a 15-16 mph on the flats ride:
This is a 17-18 mph ride: