Frontend usergroup selection

We are looking for a way to change the usergroup of a user from the frontend.
That particular field is only visible for superusers.

I know a superuser can log into the backend and change the usergroup.
But in our case we need a convenient frontend change.
We defined 3 different kind of users, and all have different access.
We only want to offer the options public, guest and registered.

Somewhere I read a post from Krileon saying:
“That's not normally something I'd recommend to allow on frontend, but it's doable combining a select fieldtype with a usergroup auto action (would require 1 auto action per select option).”

We defined a select fieldtype and now we are looking for the appropriate auto actions.
Can you help me with an example of such an auto action code?

Regards,
Patrick

You'll need 3 usergroup auto actions for that. 1 for each usergroup option in your select field. Then you'd have the following.

Global
Triggers: onAfterUpdateUser, onAfterUserUpdate (covers fronted and backend user edit)
Type: Usergroup
User: Automatic
Access: All Non-Moderators
Conditions
Condition 1
Field: Moderators
User: Viewing User
Operator: Is
Condition 2
Field: YOUR_SELECT_FIELD_HERE
Operator: Equal To
Value: OPTION_VALUE_1_HERE
Condition 3
Field: Custom > Value
Custom Value: [var1_YOUR_SELECT_FIELD_HERE]
Operator: Not Equal To
Value: [var3_YOUR_SELECT_FIELD_HERE]
Action
Mode: Add Usergroups
Groups: USERGROUP_TO_ADD_HERE

This should ensure it can't be applied to moderator accounts, only moderators can apply it to non-moderator accounts, and that the field value actually changed. With the above example lets say I have the following field.

Field: cb_selectgroup
Options
Options 1
Value: public
Label: Public
Options 2
Value: guest
Label: Guest
Options 3
Value: registered
Label: Registered

Using the above auto action example I'd configure the following.

YOUR_SELECT_FIELD_HERE = cb_selectgroup
OPTION_VALUE_1_HERE = public
USERGROUP_TO_ADD_HERE = Public

You can use "Replace Usergroups" instead of "Add Usergroups" if you want to replace all other usergroups they have as well or you can add a second action row to remove other usergroups you don't want them to have when changing their usergroup through the field.