When adding/editing attributes for a user, the 'memberof' attribute is a special case. The memberOf attribute is not an accessible attribute of the user schema. To add someone to a group, you have to add the user in the group, and not the group in the user. You can do this by accessing the group attribute 'member':
<?php
$group_name = "CN=MyGroup,OU=Groups,DC=example,DC=com";
$group_info['member'] = $dn; // User's DN is added to group's 'member' array
ldap_mod_add($connect,$group_name,$group_info);
?>