There is a new link, Invite Relatives, in the control box located in the left column of the screen.  This is a handy feature to help your relatives discover Family Cluster.

Instructions

1. Click the Invite Relatives link.
2. Type in the email addresses of your relatives.
3. Type a quick message.
4. Click send.

Additional Details

You can track your invites by clicking the Your Invitations tab on the Invite Relatives page.  There you can see three things:

1. Accepted Invitations
2. Pending Invitations
3. Expired Invitations (Invitations expire after 60 days)

If you have any questions please ask them in the support forum.

Below is an update added after original posting

Allowing Relatives Access to Your Family's Cluster

Invited relatives become registered members of Family Cluster; however, because of Family Cluster's private nature, relatives who join must be granted access to your family's cluster. 

Only Family Managers and Family Administrators can allow access.  After a relative joins Family Cluster someone, either yourself or your relative, must contact a family manager or administrator and request access.