Here is my way to harvest with friends. i hope it will work for you too!

1. Install Radmin VPN and create your network - share its name and password with friends.

2. To host: From the main menu go to   Co-op     ->    Host    ->   Host New Farm ( better not use your saved single-player games).



3. Choose your game settings:

• starting cabins: this defines how many players will be in the game with you
  
• cabin layout: will they be close to each other or bit separated
  
• set profit margin and will you skip the intro or not
  

4. Press OK
Now you have a game!

How to join the game

To join the game: In Co-op -> Join game -> Join LAN game
there  you will IP enter window

You will need to copy the host's IP in from Radmin VPN: