Automatic usergroup submission

Hello everybody!


This should be an easy question for any fusion-wiz in here.

What I need is simply for my members' names to appear in color depending on their gender. I've made the user_field already - no biggie!

Some time ago I found an infusion that does just this (working, bugless and everything) only it does this depending on what user_group you're in, instead of what gender you are. So I'm thinking: "This is great! This gives me even more control", but of course I don't want to have to manually move every single member to the gender-specific user_group (I'm counting on major number of members here), so here's what I really need:
I need the registration-page upon submission to automatically put girls into the user_group "girls" and boys into the user_group "boys".

Simple, eh? Well not for me, I'm afraid. As mentioned, I've already made the user_field that asks for the applicant's gender, and it's working like a charm, so there's no problem retrieving the applicant's gender.

Hope you guys can help me out :)