shops offer group rides to attract customers. So unless the owner is really greedy, they do not charge you a fee. Even a seriously competitive shops don't charge you a fee for a group ride.

Now if the group ride is a training ride for their racing team/club, you will be paying a fee to join their club. And they normally would insist that you wear their racing gear (kit) Jersey and pants.

Sounds like you stumbled on a racing team training ride.