trade between players?

Hello!
I have stuck to level 17...
to complete the new mission ask from me to sell 100 water with other players...
i have to invite new players from my friends or it is possible to complete it with other way?

When reached level 17, u will need to build a branch office to enable the trade options.
When u got that then u get an extra tab in the chat window, and u can trade with friends.