To add a guest/friend to your stream, you'll want to send them an invite link by clicking on the plus icon at the top left side of the editor and copying the invite link.
Note: Please see this article for information about max guest count
Once your guest visits the link they can then select the Camera, Microphone, and Speakers (Audio Source) they'd like to use on your stream.
If the guest's attempting to capture high fidelity audio with their device, they'll want to ensure they check the "Optimize for music" option under their Microphone option. This can be done before entering the Green Room or in the guest's settings after joining.
They can also choose their guest Name before they "Join the Room". Note: The guest will not show up in the Green Room of your project space until they select Join the Room. This allows the guest some privacy to get ready before joining your stream, but they still won't be added to to stream until you (the host) adds them to the project area (explained further down page).
When a guest joins your Green Room they can then do things like, Mute Mic, Hide Cam, Share Screen, adjust their Settings (Cam/Mic/Speakers), or Leave the Room with the toolbar at the bottom of their guest page. The guest can also see the Chat from the stream, the Stream Output itself, and they even have a Private Guest Chat feature they can use to talk privately with the streamer and/or other guests.
Note: Similar to how the guest must join the room on their end, the guest will also not be able to see their host, until they choose to Join the Green Room from their project space as well.
Once the guest has joined the Green Room, you (the host) can then move the guest into their project as a layer via the Add to Scene option. This will bring in the guests camera and audio source into your scene, allowing you to position/shape/size the camera how they want and make adjustments to their audio in the Audio Mixer. You can find this option by hovering over the guest's camera.