Ezekia

,If the client details are not already on the system you can add them as a new person and invite them to the portal (1). If the person is already on the system, or is already on another portal, you can select one of the existing client contacts by selecting them from the list (2):

You will then be able to amend the invitation text before sending it to the selected client.

For the client to be able to navigate between multiple portals:

The client only has one log-in to view the client portal, even if they have been invited to multiple assignments.  When they sign in to the portal they can switch easily between the available assignments by clicking the 3 dots icon in the top right corner (1), then click the Projects icon (2), and select the project they want to view (3).