Is there any way to allow external users to send a private message to a local user? I imagine this would work much like a Filedrop, but I don't see the option to mark the message as a Private Message on that page. Is there something in the group config I'm missing, or is this not possible?
For External users group it works similarly like in other groups with sending permissions. In "Admin > Groups > edit - external users > Message Settings" scroll down and enable the "Users in this group can change the Private Message option" checkbox. If the private message should be enabled by default then enable also "Enable Private Message as default" checkbox. Also in System Filedrops is possible to enable Private messaging: "Admin > Configuration > FileDrops > edit > Private Message".