MySQL problem: select a user where membergroupids = 5,7

[Yurimien]

I want to select a list of users that have a certain membergroupid

SELECT username
FROM vb_user
WHERE membergroupids = '9'
LIMIT 0 , 30

The code works when the user only has one additional group assigned, but most of my users have 5 groups assigned. The content in the database looks like: 5,7,9,10
So how can I make sure I also select those with multiple usergroups?

--------------- Added [DATE]1201523929[/DATE] at [TIME]1201523929[/TIME] ---------------
@the one that posted a reply earlier:
This selects it only if the membergroupids start with 5. But it helped me, I've added everyone to the registered members group, and expanded it to
WHERE membergroupids IN (2 , 5 , 7 )


Thank you.