Add member to a group's "member" attribute.

I need to add users to a group in FIM using Powershell.

Both the users and groups are already in the FIM MA and metaverse.
The group's "member" attribute is a reference attribute.
The list of users (accountname only) to add to a group is stored in a multivalue string called "add_to_group".
How can I update the "members" attribute with the correct value representing the correct user.

JoT