To customize permissions for a specific user in SharePoint:

1. Open the appropriate Site on the SharePoint site.
2. Click the gear icon in the upper-right corner and then click Site Settings.
3. Under Users and Permissions, click Site Permissions.
4. Choose New > Add Users.
5. Type the User/Group Name, Domain Name, and/or Email and click Add to add the user to the SharePoint site.
6. Assign appropriate Permission/s Level for the user, then click Save.