UnGroup, do what you need to do the feature, then Group the remaining
features back.
That's the case for delete..To add, i think you can just drag the feature
inside the group as long as it does not have children or references
outside. For example, Reference Patterns need to have their references
inside the group.
In Wildfire 5 (we're on WF5 now) I've seen an improvement in Groups: you
can simply delete, add , drag in or out features without the need to
UnGroup and regroup.
Hope it helps.