First of all you will need to click on "Start a new Party"
You can then share the URL at the bottom to the ones you want to invite:
You can then select the Game-mode and lobby you want to start. You can share the URL directly to that Lobby as well (in case you are a bigger party and want people to come into certain Game-lobbies)
In case you get the message below that is because one player can only create one game-lobby within a party.
If you want to play more Game-lobbies at the same time that is required more than one Pro-account. So for instance you can have 3 game-lobbies started by 3 players, as long as there is one player with a Pro-account to start the game.