No setup is required.
Simply type your client's email address to send them a secure message. You do not need to create any account for them. You need not invite them to your portal, or provide them any access code.
Similarly, to receive a secure message, simply use your upload page. Again, no client setup is required. To see how to set up your upload page, see: Receive Securely From Others.
You do have the option to create clients by importing a list of your existing contacts. This makes it easy to to send them secure messages because rather than type their email address, you can search for a contact (by typing a part of their name or email) or simply click on that contact's folder to start communicating securely. To import contacts, see: Three Ways to Import Clients.