While I don't consider myself a religious person, that just doesn't seem right to me.
What I posted:
[NAME] -- I did not request to be added to your "[FACEBOOK GROUP NAME]" group and you did not give me common courtesy of first asking me if I wanted to join. While this might be 'officially' legal under Facebook rules (and you're certainly not the first one to do it) -- the real world calls this 'Spamming'.
Much like if I were to sign your email address up to 100 bulk email services. It would be spam because you didn't authorize it. Not to mention, it would damned rude for me to do so. Your actions are no different.
Please do not add people to groups unless you ask them. Particularly if you want them to ever care about whatever message/business you are advertising.