If I have a Pro plan and add collaborators that have a free plan, do they inherit any advanced features?
When you upgrade to a Pro or Pro+Teach account, some features of your documents, such as space and file limits, carry over to your collaborators, but user features (such as spell check) do not.
Your collaborators can get these user features (such as spell check) by upgrading to a Pro or Pro+Teach account themselves.
If you have three or more collaborators and you all sign up for a year, it's cheaper to buy either group account or a teaching account together -- these provide significant discounts for multiple licenses, and include project and team management tools. See the row of options under the main plans table at overleaf.com/plans for more info.