Before I would type /join name#1234 if I didn't get an invite after joining on the app.
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Make sure your fireteam settings make them open to the public. Open up the chat window by pressing Enter. Type in “/id” (without the quotation marks), and the game will show you your Steam ID. Copy that number and send it to the other person(s). The ones joining should type “/join” (without the quotation marks), followed by the Steam ID of the person hosting.