A "special invitation" is a special request or offer to join or participate in an event or activity. It typically means you are chosen or invited in a unique way that sets you apart from others. Full definition
Imagine a feast where the poor receive special invitations! (religion-online.org)
To enable us to keep providing these services free of charge, we reserve the right to contact you with special invitations to sample or purchase private equity - related products. (pehub.com)
The catch with this system is that is a «secret» trading system that is by «special invitation only» and is offered to an elite and secret group of individuals. (7binaryoptions.com)