Help with promotion setting or do I do it differently - please help

[jazzyname]

I can really use some help from everyone,

I would like to set yo a custom fields, let call it female or males uponregistration, and when a registering members select either famale or male they are the automatically placed into that same name usergroup [w/usergroup already set up with a default gender avatar]

any ideas how I can do this, please help

Can I use the setting already available in VB or would i need a mod or an add on

thanks in advance