You can install the free Groups plugin directly from your WordPress Dashboard (recommended) or using its zip file obtained from the Plugin Directory.
Installation from the Dashboard
1. Select the Add New option from the Plugins menu on your WordPress Dashboard.
2. Input groups in the search bar … locate Groups by itthinx and click Install Now.
3. Once the plugin has been downloaded and installed, proceed to Activate it.
Installation using the plugin .zip file
1. Go to https://wordpress.org/plugins/groups/ where you will find the entry for the Groups plugin in the WordPress Plugin Directory and proceed to download the current version.
2. Check that the groups-a.b.c.zip file was correctly downloaded. If you’re using Safari and cannot get your zip file, access the General settings of the Preferences menu and untick the checkbox “Open safe files after downloading…”
3. Go to Plugins > Add New and click the Upload Plugin button.
4. Select the zip file you have downloaded containing the current Groups plugin.
5. Press the Install Now button and accept the confirmation message.
6. Proceed to activate Groups.
You can use the usual automatic update process, WordPress will notify you when a new version is available.