<human>: I [29F] run a Meetup group of about 1000 women. I've been doing it for 18 months now, and generally it's going well but it's a lot of work on my own. This is relevant because one of the people in the story, Sarah, volunteered to help me run the group recently when I sent an email to everyone asking for volunteers. Out of all those who offered, only Sarah has actually been helpful.

Sarah [20s F], who I appointed an event organiser, messaged me today to explain that she'd hosted a Meetup (drinks) and one of the girls, Jo [20s F], had turned up with a friend and they'd been rude, abusive and tried to start fights. Sarah said a number of the girls present had complained about them and she asked that I take action against Jo (she didn't give further detail).

I was surprised because I've met Jo before and did not get the impression she was difficult. I've never met Sarah. I thanked her and promised to message Jo, but decided to ask to hear both sides of the story. I simply asked Jo how the night had gone, thinking perhaps she'd admit her friend and her had got a bit out of hand. Nope, she replied that although she was nervous (the reason she brought a friend) she'd loved it, had a great time and really got on with everyone well. 

Now I'm not sure how to respond to her as I want to back up Sarah but don't really want to burst Jo's bubble. Anyone got any advice how I can respond to Jo letting her know there were complaints, but not upsetting her and putting her off coming back?

Can you give me a shortened version of the text?
<bot>: Run a Meetup group, helpful event organiser asks me to tell someone their behaviour was unacceptable. I don't want to upset this girl. How do I tell her?