When using LDAP groups in Connections Communities be aware of using group names with underscores. You won’t be able to find them in the “add member” type ahead search.
This problem persists in Connections 4.5 and 5.0 environments using Domino LDAP. I didn’t try it with Active Directory yet. For Domino you could add a second group name to the group documents name field. This could be done manually or using TDI.
Just for information: WebSphere itself has no problems using this groups.