Create a subgroup and make the people you want to add calendar items admin of that subgroup.
Then, in ACES, the subgroups calendar items appeared on the group's calendar. This functionality was lost in the port.
I'm not suggesting we replicate this rather convoluted approach. I think we ought to build a more straight forward way of changing the permissions on calendar on a community by community basis. I don't think it would be that hard technically and may even be in OACS calendar UI currently.
I also think that we should consider reimplementing having a subgroup's items appear on a group's portlet. This is probably harder.
.LRN is putting together a User Advisory Board to assemble and prioritize feature lists. You should get this issue onto thier radar.