Change Groups display at user/register

Hi - I am a relatively newbie themer trying to change the wording on the groups checkboxes at user/register from "Join [group]" to "Pick your club [group]" - have attached image of default listing to hopefully clarify.

It looks to me like I need to somehow override the og_register function being called by theme_checkbox but I cannot figure out how to do that.

Grateful for any direction - have searched all over - even searched all the og code for "Join" to try to understand where it is being accessed from (not much fun with sql queries...)

Thanks