To add users to a public group, follow these instructions.
Navigating to "Setup"
In order to add users to a public group, you must first select the gear icon in the upper right corner of your screen and then Setup.
Updating the Public Group
In the QuickFind box, start typing to find and select Public Groups.
Next to the Public Group you would like to add members to, click Edit.
From the Search drop-down menu, select the type of member to add. If you don’t see the member you want to add, enter keywords in the search box and click Find.
Select members from the Available Members box, and click Add to add them to the group.
Please Note: Public groups are used to share records with the users in the group. Ensure that you would like to update the records that user has access to before adding them to an additional public group.