First make sure your Project is a Client Project and not a Discussion Project. Clients cannot be invited to Discussion Projects. You can convert a Discussion Project to a Client Project by selecting the "Save Project As" command. Choose "Client Project" as the Project Type.
To invite a Client, fill out and save the Info tab of your new Client Project if you have not done so already. Notice the "Invite Consumer" section will appear at the top of the Info tab. Click one of the large plus buttons and begin completing the email invitation form.
You can enter an email address or click the "Invite Consumer from Consumer List" button to work with a client that you have worked with previously on Lifester. Add a welcome note and click "Send Invitation" to invite your client to the Project. Your client will receive a link by email that brings them right into your Project.