This section of the Outriders guide has information on how to play with friends on different platforms. On this guide page, you will learn where you can invite friends and how to invite players from other platforms.
To invite your friends to join an existing session, at the beginning of each location, go to the station that will allow you to invite other players, just like in the picture below (also note the icon with the three characters - it also indicates that you can invite others here).
A menu will open up, as in the picture below, where you can invite players from your friends list (on the right). You can also use random player selection by clicking on the button above your friends list. You can use TAB (by default) to change the privacy of your session (from public to private or for friends only).
In the bottom left corner, you'll find options for crossplay (cross-platform game session) with other players - you can enter a code you get from your friends, or generate your own and invite them to your session.