Update User "Field Type" for Entry in Plugin
When a User field is saved, it takes an array of user IDs only – not complete UserModel objects. Something like this should work:

// get IDs for current members of group 
$members = $entry-&gt;members-&gt;ids();

// add new user ID to members
$members[] = $addUser-&gt;id;

// update field with new $members array
$entry-&gt;getContent()-&gt;setAttribute('members', $members);


The rest of your code looks fine, except for a minor typo – $enries should be $entries.