When adding user account to a group on user Properties | Member Of tab groups from the same domain only can be added.
Other domains are not avaialble in the Select Objects dialog.
Use the the group Properties | Member tab to add users to group
Create a Managed Unit to include the required domain explicitly.
The Managed Unit will be available in the Select Object dialog under the Managed Units node.
Enhancement request TF00273832 has been submitted to Development for consideration
in a future release of ActiveRoles Server.