You can quickly adjust assigned activity groups by browsing to the Activity Groups edit page. Go to the profile page of any camp event, and then from the More drop down menu, select Activity Groups.
You will then be shown a page of controls similar to the screenshot below.
Before assigning customers to activity groups you will want to make sure you've Defined Activity Groups for this camp.
Once you're ready to modify the assigned activity groups for select customers, click the checkboxes next to their names, choose the desired activity group from the pull down menu to "Move Checked Customers To" and then hit apply.
You may need to scroll down the page, but you will then notice that the selected customers have been moved to the new activity group.