MangoLion I don't want my friend's users information to be at risk
Unless you or your friends are re-using passwords and you don't install extensions that collect excessive personal data, I don't see how this could be a risk. At worse you're leaking the email adresses ?
As said by luceos Flarum is built with high quality components and put together by good coders so I wouldn't worry too much about that. If we do discover an issue a fix will probably be quickly released as has been done in the past (beta 7.1 as example).
I'd be more worried about user/admin errors (like exposing database backups publicly by putting them in the webserver folder, like I've seen... or poor database passwords...). If the server is handled by a capable webmaster, everything should be fine, beta software or not ?