Assign a user to a parent account. Enter the user ID of parent account from "Edit Profile" screen. Assigned membership level must be a "Child" level.

pmpro-sponsored-members-assign-to-parent.php

/*
Assign a user to a parent account. Enter the user ID of parent account from "Edit Profile" screen. Assigned membership level must be a "Child" level.
*/
function pmprosm_assign_child_members($profileuser)
{?>
	<table class="form-table">
		<tr>
			<th>
				<label for="assign_child_member"><?php _e('Assign to Parent Account'); ?></label>
			</th>
			<td>
				<input type="text" name="assign_child_member" id="assign_child_member"/>
			</td>
		</tr>
	</table>
<?php
}
add_action('show_user_profile', 'pmprosm_assign_child_members', 20, 1);
add_action('edit_user_profile', 'pmprosm_assign_child_members', 20, 1);

function pmprosm_profile_assign_child_members($user_id, $old_user_data)
{	
	if(isset($_POST['assign_child_member']) && (int)($_POST['assign_child_member']))
	{	
		$parent_id = $_POST['assign_child_member'];
		$sponsored_code = pmprosm_getCodeByUserID($parent_id);
		$membership_level_id = (int)$_POST['membership_level'];
		
		if(pmprosm_isSponsoredLevel($membership_level_id))
		{
			pmprosm_changeMembershipLevelWithCode($membership_level_id, $user_id, $sponsored_code);
			pmprosm_addDiscountCodeUse($user_id, $membership_level_id, $sponsored_code);
		}
		
	}	
}

add_action('profile_update', 'pmprosm_profile_assign_child_members', 10, 2);