Adding member to secondary usergroup via SQL query

[Easelm]

I am working on an offsite script and everything is working fine, but I am
having some problems figuring out the best way to add a member to a
secondary usergroup if they are already in another through mysql query.

I understand I can simply update or insert my data directly in to the membergroupids field but I am
asking how I can do this if they are in another secondary usergroup already, since they list
separated by commas.

So what's the best way to execute a query where I can add the new groupid to
membergroupids if the member has more groups already or none?