Like all good relationships ...

Like all good relationships -- Communications, Respect and expectations based on reality.

  I am not familiar with the area and the issue you may be seeking to address so forgive me if too surface an answer -but I offer it.

The key is first getting a common understanding of the stakes of interested parties.   You must develop a common understanding of the situation/ opportunity you want to address but everyone comes from there own perspective.   So understand where they are coming from first.

Then you educate the group collectively on the value of addressing the situation collectively   That may mean educating the group collectively  - the science and facts do matter. 

Then you need to establish the common goal and set both short and long term expectations based on resources which include political realities and building momentum over time.

You must define the  direction and the timing.  If you fail to define your direction/goals then expectations can't be realistic and failure of the group is ensured. 

Good luck-