Make a user join a usergroup based on cb_field value on moderator review

I would like to achieve a simple join group after user has filled a field value (ex. checkbox)

Then a moderator can review all the users who wants to join the desired usergroup and select who will and who will not join.

Until now I've made the field, made the userlist so a moderator can view those users but I can't find an optimal way to make moderators review them, select them and authorize.

I tried using workflow module but it only shows pending users by cb system. It would be perfect if I can use workflow to show users based on cb field, select them and run an auto action based on selection.

Can you lead me in the right way to approach this?

You could automate it using CB Auto Actions, but there's no way to implement some sort of review process unless you have the auto action only execute if the person saving the profile is a moderator. What this would require is the user would check the checkbox, save their profile, they'd display on your userlist, your moderator views userlist and clicks to go to a users profile, moderator edits the profile, reviews, then saves the profile. The moderators save would give them the usergroup using the below auto action.

Global
Triggers: onAfterUserUpdate
Type: Usergroups
User: Automatic
Access: Everybody
Conditions
1: [user_id] Not Equal To [cb:userdata field="user_id" user="#me" /]
2: [cb_mycheckbox] Equal To 1
Action
Mode: Add Usergroups
Groups: SELECT_USERGROUPS_HERE

Your only other solution is to develop something to handle an application based process. You maybe able to do that using a custom form extension like RS Forms where a user submits a request that can be processed.