General:
* Interested parties
* Key players
* Influencers
* Constituents
* Participants
More specific:
* Investors (for financial stakeholders)
* Customers (for customer stakeholders)
* Employees (for internal stakeholders)
* Community members (for community stakeholders)
* Government agencies (for regulatory stakeholders)
The best synonym will depend on the specific situation and who you are trying to communicate with.