In WordPress ‘do_action’ is used to add an action hook in a plugin, which then can be used to hook our own function with plugin.
Check Codex for more details: Hooks API WordPress
To add your own function you will have to do the following:
add_action('groups_screen_group_request_membership', 'your_function_callback');
function your_function_callback($id){
//here id can be used to get the id of current group
//write your code here
}
Feel free to ask if you have any doubts.