The requests & forum posts I've seen currently are using terminology something like:
Viewer - View
Editor - View, Edit, Accept, Decline
Author - View, Edit, Add, Accept, Decline
Manager - View, Edit, Add, Remove, Accept, Decline
Admin - View, Edit, Add, Remove, Accept, Decline, Administer
Bug 15205 - share roles - custom (editor / author) levels
-Asking for an editor or author level, but an admin settable custom would be even better.
Bug 12654 - Request Editor share level between Viewer and Manager
-Just editor/doesn't include 'add' - though it's a rather common use case if you wanted to add mail items, but they wanted a more locked down calendar that you could only work in previously laid out appointments (think a doctors office, or anything with fixed timeslots etc).
In the mean time you can manually set things like the below to allow adding items but not deleting:
zmmailbox -z -m
theshare@domain.com modifyFolderGrant /Folder account
user@domain.com rwix
r = viewer rights
rwixd = manager rights
You could do any of the following [account <name> |group <name> |domain <name> |all |public| guest <email> <password>] followed by the permissions like r, rw, rwix, rwixd, rwixda, none, etc.
(r)ead - search, view overviews and items
(w)rite - edit drafts/contacts/notes, set flags
(i)nsert - copy/add to directory, create subfolders
action (x) - workflow actions, like accepting appointments
(d)elete - delete items and subfolders, set \Deleted flag
(a)dminister - delegate admin and change permissions
The createMountpoint command is commonly used after this:
zmmailbox z -m
user@domain.com createMountpoint --view appointment "/Vacation Calendar"
vacationcal@domain.com /VacationDates
I vaguely remember you used to be able to do -i on modifyFolderGrant to automatically send an invite, but haven't checked a 4.5.x box to see if that was the case - for filing a new bug against it not working in 5.0.x