You can add a user to your SharePoint site, that is not within your domain.
To add the user to your Control Panel, please refer to the following steps:
1. Log in to your Control Panel, at my.saashost.net, using your administrative log in credentials.
2. Under Users > select Add Users
3. Toggle SharePoint to ON > Click Submit
To complete adding the user to your SharePoint site, please refer to the following steps:
1. Log in to your SharePoint site with your administrative log in credentials.
2. Select Site Actions > Site Settings > click on People and Groups.
3. Click on New > select Add New Users > in the Users/Groups box, enter the email address of the user you want to add > specify Permissions for the user.
4. Click Ok to save.
Note: The steps provided are for a SharePoint only user. This user will not receive notifications.