When you are setting up the booking proposal or creating the shoot, there is an "Email Link to Shoot" toggle. Enabling this will send the client portal link to your client once the shoot is booked.
If you'd like to send it at a later time manually:
Edit a contact.
Click the "Client Actions" button in the bottom-left corner.
Click the "Email List of Shoots" button.
You can also find (and edit) the URL for your Client Portal login page on Settings > Account & Branding > Brands. The URL for the client portal shows up on the front of the card for each of your brands.
To edit the portal URL, click "Edit" on the brand and edit the "Brand's Subdomain" in the modal.