This is coming in 4.0!

You will have to create a view for this and bind the field to it. The problem is the project group as this is per project. So you will have to enumrate the user for global groups for all projects (cross join):

SELECT distinct userid, fullname, projectid FROM (
    SELECT u.userid, firstname + ' ' + surname as fullname, p.projectid FROM geminiprojects p, geminiusers u
    LEFT JOIN geminiglobalgroupmembership g ON g.userid = u.userid
    WHERE u.active = 1 AND g.globalgroupid IN(1)
    UNION ALL
    SELECT u.userid, firstname + ' ' + surname as fullname, p.projectid FROM geminiusers u
    LEFT JOIN gemini_projectgroupmembership p ON p.userid = u.userid
    WHERE u.active = 1 AND p.projectgroupid IN(8)
) users
ORDER BY fullname

Make sure you mark the custom field to filter by project id