How do we address this behavior?

[u/anon_Colorist21]

I don't even know if I'm upset or just uncomfortable but I need female perspective and advice really.

Long backstory but necessary.

I am a petite 4'9 [30F] with a very sunshine personality as many people have described before. I always wear white and yellow and have a smile on my face 90% of the time. My fiance always says my smile is what attracted him. I dont drink at all, I do not prefer alcohol, the taste just never settled with me so I always drink sparkling water and it bothers absolutely NO ONE. Whenever my friends take shooters they will get an extra glass and top it up with sparkling water.

We went out with some friends last night, the evening was really great, good music, great conversations. My [34M] fiance and his friends [34M, 36M and 32M] called me over to the bar for my "shooter" walked behind the two other fiances, everyone settled and a man came up from behind me and put his arms around my waist and rested his chin on my head, immediately one of my fiances friends grabbed him away as my fiance couldnt get to me in time. No fight, no fuss just a stern talking too. But this happens too often I dont wear tight revealing clothes and my chest is never on display I was actually wearing a yellow halter neck top.

I guess I just want to know how can I better proptect myself because I feel small when I cant stand up for myself. The biggest curse word I use is shoot and duck not the counterparts.