Group membership request does not respect group cardinality for group membership

I will check it

So, Actually, to add user to a group we are using Group::addMember and inside we have such code:

    $storage = $this->relationshipStorage();
    $relationship = $storage->createForEntityInGroup($entity, $this, $plugin_id, $values);
    $storage->save($relationship);
    return $relationship;

So, as you see the $relationship doesn't not validate the entity and that is why it allows to add user and doesn't respect cardinality settings.
it is a bug on group module :(

Thanks a lot for the fast reply and pushing the issue to the group module team.
After some more digging I realized the the native joining process does not work correctly either.
I will pass on that info to the issue you opened up on the group module issue board.