Employees and contractors agree to assign IP rights to work produced while working for MUI to the company as part of their signed contract.
No CAA or CLA is required for contributions to MIT-licensed open-source projects. It is widely understood that contributions are licensed the the project, while copyright is retained by the contributor. See Per the GitHub terms of services, a contributor who opens a pull request agrees to license their work under the same license used by the repository. for how it works legally.
This poses some difficulty if it is desired or intended to change the license after the fact, for example from open source to commercial, as prior contributions retain their open source status. MUI accepts that versions of the software (and the contributions that constitute them) released prior to any change in license retain the license under which they were originally created and released. The original contributors retain the copyright in their contributions.
Third-party contributors to commercially licensed software are required to sign a CLA (Contributor License Agreement) before their change can be accepted and merged. A license key for the product in question is offered in exchange.