Technically some of the DIKs saw Bella leave in ep 6, but they may not have have thought that was more than a one night thing. Also technically Nick saw Bella leave at the end of ep 9, but that's nit-picking. Nick (the fucking rat) spills the beans to other DIKs including Derek in ep 11, but what they think about it has not been addressed. Maya can find out in ep 10, but if she doesn't Josy (the fucking girl-Nick) will tell her and Sage in ep 11 where she herself found out about it. Jill of course found out in ep 8. And Bianca in ep 11. Bianca is somewhat special being the first and until now the only person Bella has told voluntarily. And technically Neil knows MC has a girlfriend named Isabella, but MC chickened out on telling him some key details.
And now I feel a sudden urge to start explaining how Bella's path can become the Main Girl Harem path, but I shall refrain from doing that unless I'm asked about it. Please ask me about it.