Download the latest release and Unzip the downloaded release.
Copy the dmzx/groupsintopic folder to /ext/ (if done correctly, you'll have the main extension class at (your forum root)/ext/dmzx/groupsintopic/composer.json).
Navigate in the ACP to Customise -> Manage extensions.
Look for Groups in viewtopic under the Disabled Extensions list, and click its Enable link.
Uninstall
Navigate in the ACP to Customise -> Extension Management -> Extensions.
Look for Groups in viewtopic under the Enabled Extensions list, and click its Disable link.
To permanently uninstall, click Delete Data and then delete the /ext/dmzx/groupsintopic folder.