Indeed, JForum does not interact with AD or LDAP like that. I understand the users already exist in JForum. After you have created the groups, you could add the user-to-group associations directly via
INSERT INTO jforum_user_groups ( user_id, group_id ) VALUES ( ?, ? )
where you'd replace the question marks with the userID and groupID, respectively.
You would find the relevant IDs via
select group_id, group_name from jforum_groups;
select user_id, username from jforum_users;