Howto require just one element of a form?

[eluser]wdcmatt[/eluser]
I have an html form that is 20 check boxes. (A list of user groups...)

I have to ensure that AT LEAST ONE is checked, more is fine, but any of them is acceptable. (I must ensure that each user is a member of at least 1 group... they can be members of more but must have at least one group.)

I know ci has the "required" tag in form validation is there something handy I could use to require one out of a list of variables? I know how to do this with standard php, but was just curious if there was a nice feature of ci that could manage this for me.

Thanks in advance.
wdcmatt