From your Client Users, to the right of filter by search and above the list of users in your Client is a button saying 'Invite new user to 'YOUR CLIENT NAME' - click this to invite a new user to your Client.
This user must be new to Siteglide in order to be invited here.
Once the Client has its first user, your Client can add others themselves.
These users will have access to only the sites created within this Client.
Once clicked, enter in their name and email address and confirm by clicking the blue button in the modal.
You have now invited a user to your Client!