how to facetime using a computer

I suspect you have a Windows computer, so let's start with that option.

Windows computer: Join a FaceTime call from an Android or Windows device (This link is also at the top of the page).

You will need a link from the organiser of the chat to join the conversation.

Mac: Use FaceTime on Mac

Macs have the FaceTime application and can join or start a chat with that software.