From your site 'Users' to the right of filter by search and above the list of users in this site is a button saying 'Invite new user to 'THIS SITE NAME' - click this to invite a new user to this site.
This user must be exist within Siteglide in order to be invited here.
Click here to find out how to invite a new user to an Agency
Click here to find out how to invite a new user to a Client.
Once clicked, enter in their email address and confirm by clicking the blue button in the modal.
You have now invited a user to this site!