how old is Brianna ? Have you tried talking to her about why she does it? Or what about playdates with just one other child.. and maybe building it up to more? or maybe spending time with her talking to her while with other kids so she knows she's ok?
I also agree with the PP... she must be ok with your youngest? You might want to talk to her doctor for some suggestions as well.. they might have other parents with the same issue.
((hugs)) keep us updated :)
