If they are your friends then when they come late just say flat out my closing time is 5:30 and I really need to to be here by then as I need to get dinner on the table and sometimes I have made plans to go out as soon as I'm finished work. Also mention you didn't say anything before because you didn't realize that it wasn't gong to be a one time thing. This is one of many reasons I don't have family friends or neighbours as clients