Which is the MOST important stakeholder group that a service provider needs to collaborate with?

The first important step is identifying and managing all the stakeholder groups that an organization deals with. The first and most obvious stakeholder group is the customers, as in service management the organization’s main goal is to facilitate customer outcomes. Other examples of stakeholder collaboration include:

Developers working with other internal teams

Suppliers collaborating with the organization

Relationship managers collaborating with service consumers

Customers collaborating with each other

Internal and external suppliers collaborating with each other

The contribution to improvement of each stakeholder group at each level should be understood, as should the most effective methods to engage with them. Depending on the service and the relationship between the service provider and the service consumer, the expectations about the level and type of collaboration can vary significantly. It is important to involve stakeholders, and address their needs at all levels. Determining the type, method, and frequency of such messaging is one of the central activities related to communication.

https://www.bmc.com/blogs/itil-guiding-principles/