Hi, I am looking to work on a single site with someone else. I plan on buying the CMS site plan, which allows for 3 content editors. But do I still need to buy a workspace plan in order to get them into the workspace so that they can edit the site? Or can I just invite them from their completely free account.
Welcome to the community @jhviper! 
If you’re wanting to invite someone to collaborate in the Designer (where they can manipulate the actual project design rather than just content) you’ll need to invite them as a “guest”:
Right now, guest access is limited to people on either the Freelance or Agency plan, so they won’t be eligible if they’re on any of the “In-house team” plans (you can find a breakdown of the differences for each plan here).
Hopefully that gives you the info needed but don’t hesitate to reach out if you have any other questions!