You can add and remove students from groups! Here's how to do it.
Go to your groups. From the K12 dashboard, go to the "Link" button, then the K12 LMS. From there, click "Students" in the top left, then "My groups."
By the group you want to edit, click the "Action" button, then "Edit Group."
Click "Select Members" in the top right. Search for the people you want to add and add them by clicking their name. To remove members, click the X by their name.
Click "Save and Close," then "Update Group."