Hi homenba, good luck with your search. I spent two seasons at a polo estancia, but in the guest relations part of it; like someone pointed out, that is usually an easier area for foreigners to get in. I contacted 15 estancias before I came to Argentina and got offers from three, but they were all in guest relations. There are some female grooms in Argentina, but few, and as far as foreign grooms go, I only met one girl (from England) and she was working for an English polo player and came here with him.
In most polo clubs, there are only some of the grooms that are hired by the actual club, the majority are hired directly by each player/member of the club, so each player or member is responsible for hiring the person, training them the way they want to, paying them, etc. And most grooms tend to be young and without a family, they often live on-site in groom housing, so unfortunately having a family would not be a positive really.
If you still want to try, you should try to find people that are members of polo clubs and just start asking around. Or, if you know of a place, maybe you could start going as a guest and just ask to shadow some of the grooms? It wouldn't be paid but if they let you you could learn a lot.
Best of luck!