To add or remove Stakeholders in a Stakeholder Group, first open the Group in the workspace by firstly clicking on its name from the Manage Enterprise or Manage Project window.
Next click on the + button in the stakeholders dock to the right.
This will open up the 'Assign Related Stakeholder' window, and you'll see three tabs across the top; Remove, Assign, and Add New.
To add an existing Stakeholder to your Group use the 'Assign' tab, search for the Stakeholder or group of Stakeholders and click the green + beside them. In order to add the entirety of your search results, click on the bold green +.
To remove Stakeholders from your Group, click on the 'Remove' tab, search for the Stakeholder/s you wish to remove, and then either remove a single Stakeholder at a time by clicking on the red X next to their name, or bulk remove all Stakeholders in the search by clicking on the bold red X.
For more information on how to Create and Delete Groups/Lists and Types, please refer to this article: Creating and Deleting Groups and Types