You may add a user to your SharePoint site, that is not within your domain.
Note: The steps provided are for a SharePoint only user. This user will not receive notifications.
To add the user to your Control Panel, please refer to the following steps:
1. Log in to your administrative Control Panel, using your administrative log in credentials.
2. Under the Users section on the left, click the Add User button.
3. Fill in the required information and Toggle the Quick Add SharePoint switch to ON, then ClickSubmit
4. Log in to your SharePoint site with your administrative log in credentials.
5. Select Site Actions > Site Settings > and click on People and Groups.
6. Click on New > select Add New Users > and in the Users/Groups box, enter the email address of the user you want to add, then specify Permissions for the user.
7. Click Ok to save.