Getting mail recipient ID rather than current user ID

I have a custom email function in functions.php, that when an admin approves a user sends an email to them notifying them of this. However I need to add some custom data for each recipient, that I can add if I can work out their ID.

So this code delivers the current user's ID (the admin that triggers the send): $user_ID = get_current_user_id();

Does anyone know how I would get the actual recipient's ID?

Thanks.