Once you have created a Paper, you can simply share the URL (web address) with as many people as you want to join the Paper.
You can then have video calls, upload documents and images, write and create content together in real-time.
When there's more than 1 user in a Paper, the collaborate menu button in the bottom left will turn green and will indicate the number of users on the Bitpaper. I.e in the example below, 5 users are connected to the Bitpaper.