You and your friend download Radmin VPN, create a network, and let them join it. If you are hosting the game in Radmin, there is an IP address. Copy it and send it to your friends. Go to a private match and start the map you want to play alone. When the game starts, your friend can connect by hitting the console command or the Grave key ~ and typing CONNECT then the IP you provided them.
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denzzdrain
@denzzdrain