Selectable User Group in Login.Register form

But can the &usergroupsField property be manipulated somehow so that the user sees the groups as options from which to choose?

If this is possible, then it'll probably beat any other method.
So instead of having multiple register calls with specified usergroups attached, a single call is made with drop-down or check-box options.

Also, anyone know if the above posthook and

<select name="product" id="product">           
    <option value="Usergroup1" [[!+product:formitisselected="`Usergroup1`]]">Usergroup1</option>
    <option value="Usergroup2" [[!+product:formitisselected="`Usergroup2`]]">Usergroup2</option>
    <option value="Usergroup3" [[!+product:formitisselected="`Usergroup3`]]">Usergroup3</option>
</select>

will work in UpdateProfile, so that a user may change or add self to another group at will?

$userObj = $modx->getObject('modUser', $userId); // please provide $userId
/* create membership */
/** @var modUserGroupMember $member */
$member = $modx->newObject('modUserGroupMember');
$member->set('member', 0);
$member->set('user_group', $usergroupId); // please provide $usergroupId
$member->set('role', 1);
$userObj->addMany($member, 'UserGroupMembers');
$userObj->save();
$modx->user->loadAttributes(array(), 'web', true); // usergroups reloaded