Using Office 365 GUI are you trying to work out which groups a user belongs to?
A quick search on google reveals that folks have suggestions using powershell and exchange 2010 - which is not always the most straight forward for admins.
On the Office 365 admin GUI, its not actually immediately obvious, but it is in there!
We don't blog nearly as much as we'd like, but I thought this one would be as appreciated as much as the "how many devices can I sync to my iTunes account" blog and "Entourage root certificate" blogs.
So on with it then; sign in to office365.com as an admin user.
Under Exchange - click on Manage
At the top left Mail > Options > Manage My Organisation
Click the drop down arrow and chose another user.
Enter users name - go - you know see the Office 365 exchange Outlook Web App screen with the message above "admin working on behalf of username"
Click on Groups - and now you see the groups the user belongs too.
Thanks for reading - links to this article as always appreciated.