In Massachuessets, if you serve alcohol to guests who are or appear to be intoxicated, you could be liable for any injuries they cause to others or damage they cause to other's property. As long as you are not charging in any way for the alcohal, you should be able to provide it or allow others to bring it; BYOB may make you somewhat less likely to be potentially liable for injuries/damage caused by intoxicated  guests, but if you are performing any "bartender" functions, even with alcohol provided by others, you can't entirely escape the possibility of liability. Of course, you need to comply with all underage drinking laws--i.e. don't serve anyone under 21.