When a client books online with you and digitally signs their order confirmation on the last step of the booking process, they are then presented with the option to create a password so that they can log in and manage their booking.
If you need to send the client or staff person their login info, you can do so by going to Manage - User Accounts, then search for the user and click on "Show User" when their user card is displayed.
In the right sidebar of the user's account page you should see a Quick Send button; clicking that will open a popup window with an email template that you can send to the client that will give them access to their client portal. Just click the Send Now button to send the template as-is.
If you prefer to edit the template before sending, you can access it by going to Manage - Automated Messages and clicking on the System Templates tab:
Click on the Edit button to the right of the Customer Invitation template to edit it. You can add a signature by copying and pasting the {{brand_signature}} field from the sidebar on the right.
The email includes the {{invitation_link}} to the client portal by default. You may add as many additional fields as you like.
There is also a Staff Invitation email template that you can use to invite a new staff person to join their user portal.