Adding users to guest group via occ or provisioning API should not silently fail

Steps

1. Enable the guests app
2. occ user:add test1 + enter password. This will create a regular user.
3. occ group:adduser guest_app test1
4. curl -v -u admin:admin -H "OCS-ApiRequest: true" -X POST https://host/ocs/v2.php/cloud/users/test1/groups -d "groupid=guest_app"
5. occ user:info test1

Expected result

Both the occ command and curl should fail with an error.

Or the user should become a guest, but that would be more complex.

Actual result

Both commands report success but user1 is not added to the group.

Versions

NC 21 git master fda6ffc866cf8c5d3579fe95d1731ab747894002 with guests app git master 15087861f4dd8d707c259dd7cb5cbebb11247960

This could also be a server issue...

Context

I was trying to write an integration test in the talk app and made the mistake of trying to add the user to "guests_app" group using the provisioning API. Since it didn't fail it took me a while to notice the problem.

Start by reproducing the behavior with occ group:adduser guest_app test1 and the provisioning API request described in the issue, then inspect how guest-group membership is handled by those entry points. Done means the commands no longer report success while leaving the user unchanged: they either return an error or make the user a guest as agreed.