To add more users to your Overdrive site:
Once created, it may take time for some of the information to sync from Google - this info is not editable. You can edit each user to give them permission to change settings on your site (Site Settings) or make changes to a page (Page Settings).
Depending on the Security Setting you have chosen, the invited user will either see the whole of your site (Access All Areas) or only the folders you have given them access to in Drive (Google Drive). To open and view/edit any files each user will need to have the relevant access in Drive, if the user does not have access the file will open with a Request Access option.
For more information on Users, visit Reference/User Settings.