There are three main ways to add people to a group.
From the person’s profile
From the group’s page
From an import file
From the Person’s Profile
From the person’s profile find the groups heading in the side bar and click the ‘Add’ button.
In the window that then appears select the group you want to add them into.
From the Group
When viewing a group you can click the ‘Add People’ button on the top right corner of the page.
After this type in the name of the people you want to add into the group and press the ‘Add People’ button when finished.
From an Import File
You can add people to group by creating an import file and uploading it to People -> Import.
Here’s an example of how you could format your import file. Simple separate multiple groups with a comma.