Your horse is used to being ridden alone in a field. You are asking her to work as one of 5 horses, in an indoor, while someone is schooling barrels.
Personally, I wouldn’t do that. You are asking so much of your mare. Let her get a bit more settled with a few horses before you make her tolerate barrel racing.
I say that because I board at a facility with reiners and barrel racers. No way would I expect my dressage horses to share the ring with them, it’s just too much to ask.
Start a bit smaller and build up to the barrel racing