Hi @pursuit
Thanks for the question. This is something I have not yet documented, but WP has!
Just like the WP_Query class, there is a WP_User_Query class which you can use to search for uses based on args.
One of the args is for a custom field query. Here are some links to get you started:
http://codex.wordpress.org/Class_Reference/WP_User_Query
http://stackoverflow.com/questions/12392847/wordpress-user-query-order-by-meta-value
Hope that helps.
Thanks
E