I had a game night last night and two of my sister's friends came and played, one is male and 20, call him B, and the other is a female ~18 years old, call her L. 

When my husband was finishing his long game (magic the gathering card game), B and L still had yet to leave or text their parents and tell them to get them (they had no transportation of their own and bus is unreliable, only option is having a parent get them). 

I was feeling annoyed that they had not realized that they were slightly overstaying their welcome at our place. I teasingly said "what are we gonna do with you?" once my husband's friends had left. That triggered L to text her parent and have the parent come get them. I wanted to tell them the next time we interact that I wanted to help them realize that when they overstay their welcome it annoys other people and makes them less welcome the next time some event is happening. 

Note: I do not mind them both, they are both very sweet people and great friends to my sister. It is that my husband and myself need our space and we were already very tired from running the game night and his game had gone on for 3.5-4 hours already. When the game ended, there was at least 30-45 minutes of idle chatter while the guests were cleaning up, gathering cards, commenting on game events, talking about the next time they want to play, etc. L and B had time to realize that they could ask their parent to come get them. 

**How do I approach educating the two people about not overstaying their welcome WITHOUT alienating the friendship we have already?**