YES! it's possible! (Here's how to proceed if you play on PC)
To work, everyone in team need a "Xbox Live Active Account" (https://www.xbox.com/live ) (both Xbox and PC can have one)
1) Log in Xbox Live (then you can close the web browser)
2) Enable the Xbox Game Bar on your PC (Settings->Gaming->Xbox Game Bar) (put it on "active") then close the settings window.
3) Start Sea of Thieves the normal way.
4) Once in game, press "Win+G", that will open an interface over SOT.
5) On the Top-right Corner you have "Xbox Social", Search for the names of your friends, then add them to your list.
6) Then, right-click on the name of your friends and click "invite to game->Sea of Thieves"
7) BINGO! You can now play with your friends regardless of where you bought your game (Windows Store or Steam)
Point is : Use your Xbox Live Account (Game Bar) to invite your friends instead of Steam to play SoT.