Pretty much the same as the person above me said. The green ladies are usually family members as the sisters or girl cousins of the groom's side. Her main duty is to follow the Sil everywhere.
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Ahh. I was a green lady before. I've got asked to do it once but on my second time I said yes. Actually, I was kind of forced!
I get a call from my Dad and he says for me to get my butt up because I got chosen to do it. Which was really unexpected because I thought one of the groom's sisters was going to be the green lady! So yeah my duty was just to follow her everywhere she went! Oh my dearest! It was a pain! I think my Sil did it on purpose and tried to test me because she was literally speed walking everywhere! If she got told to help clean the table, I'd help her. If she got told to do dishes, I'd help her. If she just sat around and gossip with the girls, I'd listen and gossip too! And etc, you get the idea.
My job is to make sure she doesn't take off with another man. As mentioned above. Though we did took a walk around the block with the girls to go smoke and hurriedly came back.
I got paid shiity. Literally. I got $40. Though I wasn't surprise. Its whatever. I didn't care for the money, I just cared for the once in a lifetime experience! I don't know if I'll ever get the offer to do it again!
Though I'm just warning you. Make sure you choose a green lady who can drink. And when it comes down to the wedding a lot of people will be going at her. Well you should know that. Make sure she drinks enough to show respect but don't overdo it and become wasted. Otherwise she'll become a joke/weakling to the family and stories will go around.