What is the simplest way to share react components between projects?

Option 1: You can use npm and use private packages so they're not external facing. There are also artifactories and scoped packages that usually represent company-wide projects that can be public or private. See https://docs.npmjs.com/private-modules/intro and https://docs.npmjs.com/misc/scope.

Option 2: Essentially, you can develop projects with a flattened structure. You can then import various projects and/or components into other projects or folders. This is entirely dependent on your codebase and configuration. With this model though, a lot of times publishing to npm comes fairly naturally since each folder may be its own project with its own package.json.

You could also upload them to a private git repo such a Github and then pull them in from there.

Ryanve has a nice example over here: https://stackoverflow.com/a/28729646/1592783

Check out Bit:

Bit is an open-source cli tool for collaborating on isolated components across projects and repositories. Use Bit to distribute discrete components from a design library or a project into a standalone reusable package and utilize it across applications.