When you view your list of Group Activities you now have the option to edit, remove and remind staff regarding a particular activity. Click the "Edit (#)" link under the "# Attend." column in the row of the activity you want to affect.
|Group Activities List|
You should see a page that looks something like the one below. Staff you've already included in this activity are highlighted in yellow, and if you roll your cursor over each row you will get the options to Remove this activity from, or send a reminder e-mail to, that particular account.
|Group Activity Attendees Edit Screen|
And here's a video demo:
Jay @ Greenbush.