Google is in the process of rolling out a new version of Google Groups. The new interface may not yet have all of the features of the old interface. Until that happens, you may have to switch between the "old Google groups" and "new Google groups" to use specific features.
If you haven't done anything with Google Groups, you should be automatically setup for the new Google groups interface. However, read below to tell which one you are using and switch between the two.
New Google groups
If you see the following message at the top of the page, you are currently using the new Google groups.
To switch back to the old Groups interface, click on the gear dropdown at the top right and choose "Revert to the old Google Groups".
Old Google Groups
If you see this message at the top of the page, you are using the old Google groups. To switch to the new Google groups interface, simply click on the "Switch to the new Google Groups" link.