(MySQL+PHP) - Users belonging to multiple groups?

I have a basic database consisting of an Articles table, Users table, and Groups table.

In groups there’s a dancers group and musicians group. My users are in only one of these groups, meaning I have a column which assigns a group id to the related group in Groups.

Ok, but I now have users that are both dancers and musicians! And hence need to see the articles related to both groups. How can I associate one user, with two groups? Do I set this up in mysql or do it in PHP? I’m thinking the former, cos there can be any combination of users with multiple groups.

Any help will be really appreciated.