To add an owner or moderator to the members list of a forum, send an Atom entry document containing the new member to the access control list feed. Use the <snx:role> element to specify which role you want the person to have. You can only add one member per post.
Forums are public, so anyone who can authenticate with IBM Connections can access a forum. Therefor, there is no member level role in the members list of a forum, only owner and moderator roles.
See Authenticating requests for information about how to authenticate the request. Only forum owners can add people to a forum as owners and moderators.