How to add a user to a SharePoint site
As owner, or user with full control permissions, you can add, edit or delete users for your SharePoint site..
Step-by-step guide
Go to your site.
In the top right corner, click Settings .
Click Site Settings.
Under the Users and Permissions category, click Site Permissions → Permissions → Grant Permissions.
5. Select a group to add permissions
6. In the given Invite people to field, enter the user's email address or username and select the appropriate user from the drop down menu that appears.
7. Click on Share