If a group has already started, wait until future session dates pass and go into Manage Groups --> Details and use the EDIT GROUP button to show all the sessions and delete the session that did not take place. If the group restarts, the remaining sessions will be there for you to track attendance in. If you choose to cancel the group instead, attendance records will be saved but you will not be able to reinstate the group if it resumes.
If a group hasn't yet started and will not begin at all, cancel or delete it on the Manage Groups --> Details page. Cancelling will keep the group in your list, deleting will get rid of it entirely. You cannot restore a group once it is cancelled or deleted.