In the left panel that displays your list of Contact Groups, tick the box next to the Contact Group you are editing. The details of the Group will display in the middle panel.
Click on the Link button (and a Stakeholder Search box will appear)
Use the Search fields to search for individual stakeholders, or search for an entire group of stakeholders using the Contact Groups filter
Select the stakeholders you want to add to this Contact Group
At the bottom of the page click on the “OK” button
The stakeholder(s) will now be added to that Contact Group