You've 2 options. You can use CB GroupJive Files integration or you can use CB Gallery to have a multi-media gallery for groups.
For CB GroupJive Files it should be just a matter of installing and publishing it and the tab will be available.
For CB Gallery you'll need GJ, CB Gallery, and CB Auto Actions installed. With all 3 published navigate to CB Auto Actions > System actions and publish the "CB Groupjive - Gallery" system action. You should then see a Gallery tab within your groups.