Tax Professionals and Firm Admins can send invitations to their clients to join the software by clicking on the "Add Client" option from the Admin Portal page. Fill in all the information (pictured on the screenshot below) to create the client's account. Once you finish and click "save", your client should receive an email from Lukka to activate their account.
You can also go to the "Users" tab on the admin page to manage all the users you have created. You have the ability to reset passwords, reset MFA, and activate and deactivate user accounts.