In the current version of the game users can't send invitations to join a Cartel. If you would like your friends to join your Cartel, you should contact them and let them know your Cartel's name. Thus, your friends will be able to find your Cartel in the game and join it. Also, to recruit new members, you could go to the correspondent section on the forum at: https://www.ftxgames.com/forums/forums/cartelrecruitment/
How do I invite friends/new members to my Cartel?
Tilting Point Support -