We all have heard about the forbidden fruit effect i.e., almost everyone wants something that they can not have. People want to feel "exclusive" and a part of an "inside group."
But how can you leverage this behavior to increase the chances of success of your joint solution?
It's easy.
When you create a New Joint Solution - send out exclusive invites to selected partners before general availability.
Give early access to your regional leads and partner field teams; once they accept the invite, onboard them to your Joint Solutions and mutual goals. Once aligned on the goals, give them access to the latest and greatest sales collateral.
This way, they would feel exclusive, inclusive, and could act as cheerleaders for your new joint solution.