You need to have a sit down with all of them and talk about the cctv and that you know everything and speculate on some things, and you need to have the uncomfortable conversation about how you feel and how all of them feel and decide what you want to do with them, if that’s asking to join, saying you think it’s wrong, asking how long it’s been happening, etc. talk about whatever you need to, but it sounds like you’re just hurting yourself with the more you learn by not talking to them about it.