As an author of an entry, you can add contributors to your posts. Each of these contributors can edit the entry but cannot add additional contributors. Only the primary author can add new contributors.
At the moment, contributors can edit posts sequentially and not simultaneously. In other words, only one contributor can edit the post at a time.
To add contributors to a post, you'll need to first add contributors to your publication by going to the contributor settings page (https://mirror.xyz/dashboard/settings/contributors) and adding contributors from their Ethereum address or ENS name.
Once that's done, you can add contributors to an entry by going to the edit entry page and clicking the gear icon in the top right, then scrolling down to the bottom until you see Byline Contributors. From here, you can select which contributors you'd like to add to the byline.