The shared ambition is the share image of a desired future - your "I have dream!"
A possibility to find it could be the "newspaper exercise" - I have used this successfully in the past - by writing a headline or even a small article about your organization 5 years in the future. What will the article be about? How will people talk about the organization? What will the organization look like? What will it have achieved?
The important thing about ambition: Break....it....down !!! Ambition has to be - as the the word says - ambitious. There will be setbacks and questions if you are on track. When you set small, visible goals and people achieve them, they start to get it into their heads that they can succeed.
My professor would have a football analogy now...but everyone who knows me, knows that I don't know anything about football ;).