When you send a secure message an email notification informing them about the secure message from you is sent to your recipient.
No separate account invitation is sent. In fact, invitations are not needed in Encyro. You do not need to first invite a client or create their account. Just send a secure message to their email address as if they already had an account. You do not need to care if they sign up or not. Everything else is managed automatically for you.
You can create client folders by importing your contacts as well. See Three Ways to Import Contacts.