This will require the PCs to be on to allow access. Might be useful as a temporary thing, but it's individual as far as I can tell (documentation is only available after install) so we couldn't check each others' diaries.
I now think I'm heading towards Groupware - a step up & lots of work, but I can set it up on a server (or leave my PC on 24/7 & run in VMware) then we'll have some useful extras. I'm quite prepared to pay for an appropriate solution, but obviously haven't got an Enterprise sized budget. I was hoping maybe there was a solution that was a halfway house between a consumer app & a full-on Enterprise system. Such is life.