When selecting correct filters in
https://www.lfs.net/online/activity there are currently 37 demo servers that are not marked private or cruise,yes,most of them are empty - as long noone joins them,noone joins them,the weird cycle noone likes to break. Just join a server with a combo you like and get yourself some warm up - eventually people will join,with some luck you might end up with a full 12-car grid (max in demo).
I would suggest AA or MRc servers - both are active teams,however I have to admit I have no clue how activelly the servers are admined. Additionally there is GD server,which hosts month long competition where you can even win licence if you're good and active,info here:
https://www.lfs.net/leagues/670/season/862