When your guest joins your project using the invite link you sent them, they will be automatically prompted to allow access to their microphone in the upper left hand of their browser window.
In Chome the prompt will look like this:
In Firefox the prompt will look like this:
If they accidentally do not allow access to the microphone. Simply refreshing their page will re-prompt the microphone access. If they unintentionally block access to the microphone, they will then need to allow the microphone access in their browser settings.
In Chrome this is relatively easy. There will be a camera icon with a red x on it in the right side of the url bar. They can click on this and choose to allow the microphone. They will then need to refresh the page to have that setting take effect.
In Firefox there will be a lock icon in the far left of the url bar. Click on this and it will let you allow the microphone access in a dropdown setting. Then refresh the browser tab and the microphone should be active.