Humankind is a grand strategy game, and while you can certainly play with the AI, you can also play with your friends. You can play with multiple friends in Humankind and you can either host a game or join a game that your friends created.
To play multiplayer with your friends you simply have to go to the multiplayer tab on the main menu. This brings up the multiplayer menu where you will see a list of current games. From there you can either create a game or search for your friend's game with its lobby ID.
When you create the game you will have several options that you can adjust and choose from. You can select how many players will be in the game and each player can choose their avatar. You can change the size of the map as well as what kind of map generation it will have. Other options include the pace of the game which controls how many turns there will be and the difficulty of the AI. You can also choose from premade maps if you have one that you like.
Humankind offers crossplay as well. This is only for Steam and Xbox on PC. Google Stadia players will not be able to play with those on Steam or Xbox.
For more information on Humankind, check out What are and how to complete Deeds in Humankind and How to improve stability and stop a Revolt in Humankind on Pro Game Guides.