Hi
Yes, in demo you can play multiplayer.
In menu click Multiplayer - Display list of hosts
Then you and friends decide on a server and all join there.
Or click "Start new host" to start your own server. (if you want to be able change settings like number of laps, cars etc)
If I remember correctly demo includes 3 cars and 2 tracks and multiplayer is limited to 12 drivers.
There is no other limit (no timelimit or anything like that)
If demo is useful to "get impressions", I am not sure. The demo races are usually short (5 laps or so) and can be fun, but also a bit choatic sometimes.
And of course some things of simulation like tire wear will not matter in so short races..
(In full version you can join leagues or longer races with more cars and longer duration)