You may want to look into the commands /detach, /take and /away. I not at all an expert (At least not now), but they do what they names indicate except /away, than puts your user under control from AI.
Corbeau was talking about Greatturn, which is different to the regular freeciv servers. You can't get any nation in a Greatturn server just typying in /take. The first thing you need to play is to introduce a password.
As there isn't a delegation system through the web, maybe Davide can manually allow a player to control another nation. I don't know if it has happened yet. There is another way, but it is a very bad one, using password sharing.