Cat, he's saying he's gonna buy the full version for them.
Anyway, Fatty, adding it to Steam only allows you to use the Steam overlay via the Shift + Tab method, it doesn't really do anything other than that.
Our server has no "yard mod" or claims plugin, but as we're whitelisted we don't really have to worry about much. In theory, nobody will mess with other peoples' stuff. It's also not a hostile server, meaning nobody will go in there and kill them and steal their stuff, only mobs will attack them.
As Dude posted before, everything is saved on the server and will be there every time they rejoin.
Another plus of our server is that we're running Dynmap which will allow them to see where they are in relation to anybody else on the server.
If you end up getting them accounts, we will need to add them to the whitelist so that they can join, but beyond that they should be good to go.