If you have a contact and you would like them to be able to view multiple client portals, you'll need to re-create their contact card per each client they should have access to.
You will have multiple contact cards with the same email address. You'll need to add the same contact email to each client.
As you create the contact, make sure that you toggle on the switch that grants portal access.
When your contact logs in to the client portal via your link (which can be found on the homebase > focus tab), they will have a switcher in the lower left corner when they click on "Settings."
Here is what the client portal will look like for this contact: