First, navigate to the 'Users' tab of Site Settings for the site you would like to invite an existing user to.
Next, click the blue button button that says 'Invite new user to 'THIS SITE NAME' - click this to invite a new user to this site. You can find this button to the right-hand-side of the 'Users' page.
Once clicked, enter in the email address of the existing Siteglide user and confirm by clicking the blue "Invite existing user to..." button in the modal.
You have now invited an existing user to this site!