The "Selected user's membership" list does not appear when selecting a user in the Potential Members list. This appears to be because the user selector has not been properly implemented:

• init_add_remove_members_page is being called (in group/members.php, at the end) using $PAGE->requires->js_function_call instead of $PAGE->requires->js_init_call, so it does't get Y passed in to it
• M.core_user.get_user_selector is also not present, as user/selector/module.js has not been included
• the user selectors do not appear to have been initialised correctly from the JS side (M.core_user.get_user_selector('addselect') returns null even when the above two points are resolved)