I'm looking for a good word to refer to use as a label for a list of organizations that one's organization must coordinate with, but are not considered partners. Such as the employee's union and watchdog groups. Everything I've been able to come up with (cohort, for example) implies chumminess.
Works well for your intended usage. Having worked in private and public sector, I can attest that the word is used to refer to groups that are connected to business interests / projects, but not directly partnered with the business/organisation.
Here are some definitions from boundless.com:
A stakeholder is an individual or group that has a legitimate interest in a company.
A corporate stakeholder is a person or group who can affect or be affected by the actions of a business.
Internal stakeholders are entities within a business (e.g., employees, managers, the board of directors, investors).
External stakeholders are entities not within a business itself but who care about or are affected by its performance (e.g., consumers, regulators, investors, suppliers).
The employee's union in your example would be an example of an external stakeholder.