The easiest way to link multiple stakeholders to a contact group (if the stakeholders already exist in Simply Stakeholders):
Navigate to the Admin menu (the little gear icon in the top menu bar)
Click on Contact Groups
Tick the box next to the contact group you'd like to add stakeholders to
Click on "Link"
Search for stakeholders using filters (or just click on "Search" without entering any filters for a full list of all stakeholders in Simply Stakeholders)
Tick the box next to the Stakeholders you want to add to the Contact Group
Click on "Ok". All the selected stakeholders will now be added to that contact group