Currently, the --reviewer only takes account identifier (email) as parameter. It could be updated to take a group name as argument.
My use case is that we have groups corresponding to our development teams. Developers often want to add their whole team as reviewer for their changes. Currently they need either to add --reviewer manually to their receivepack command (and keep in sync with team changes) or go to the web interface to add the whole group at once. Having the ability to add the group directly from the receivepack would ease the process.