As a Google Group Manager, you should be able to modify and view the members of your Groups. If you can't, you will need to make changes to the Group's permissions to give you these abilities.
- Go to https://groups.google.com/
- Click on My Groups
- Click on Manage Group under the Google Group you manage
- Click and expand on Permissions on the left-hand side
- Then change Access Permissions -> View Members -> to include Managers of the group
- Then change Moderation Permissions -> Manage Members -> to include Managers of the group