Single speed, not a fixed hub? If you don't know how to fix it, yes absolutely take it to a shop. Stuff can happen on group rides, which is the reason that our group has admission standards. One can't just show up and ride. If you're worried about rep, get your bike fixed and explain/show what happened and the fix to the ride leader. Don't worry about the rest of the group - they'll be OK after a a few perfect rides. Mechanicals happen.
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Results matter