cannot add users to guest_app group

[individual-it]

Steps to reproduce

1. create any user
2. try to ad the user to the guest_app group (via user management app or API)

Expected behaviour

ether error shown or user added to the group

Actual behaviour

response comes back with 200 but nothing happen, no error but user not added to the group

same happens

Browser log

[cortho]

Looks to me like the same issue as in owncloud/core#35289

[pmaier1]

In the case of the guests_app group I thought this was expected behavior as this group membership currently doesn't give you anything. A user will not become a guest by adding them to this group. Generally, this is an issue which needs fixing, of course.