Ok, so it appears that someone knows your email address. That, in itself, is not unusual, but it is sort of random that they would want to invite someone they don't know to join their family plan.
Apple does provide a means to be removed from a Family Plan, but I'm not sure that the following article covers this particular case. (Note that I'm assuming that you don't share your Apple ID with anyone or that it has not been compromised.)
Ref: Leave Family Sharing - Apple Support
What I suggest that you do before this is resolved are:
- Immediately change your Apple ID password.
- Contact Apple Support directly for assistance. You can get to them by using the "Get Support" link in the upper right-hand side of this forum's web page.