Yes, using CB Query Field you can create a Query Select fieldtype that pulls its select values from a database query. Your database query could be something like the below to pull a list of users.
Query:
Code:
SELECT u.`id`, u.`username`
FROM `#__users` AS u
INNER JOIN `#__user_usergroup_map` AS m
ON m.`user_id` = u.`id`
WHERE m.`group_id` = GROUP_ID_HERE
Value Column: id
Label Column: username
With the above you'd replace GROUP_ID_HERE with the actual id of the user group id.
krileon
