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1. We all went to the concert together in one group.

2. We all went to the concert together in a group.

I heard American saying #1(in one group).

I wonder if the meaning changes slightly if I say "~ in a group."
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moon72962. We all went to the concert together in a group.
Neutral focus. No implicit contrast. in a group is really redundant because together implies in a group.

moon72961. We all went to the concert together in one group.
Focus on one. Implicit contrast with statements such as We went to the concert in two/three/several groups.

CJ
Comments  
It could be the difference between everyone travelling together, or meeting at the concert.

The article "a/an" means "one".
 CalifJim's reply was promoted to an answer.