The easiest way to achieve it is to create an empty project and using the "Share Project" dialog in the "Add Teammates" tab, add as many users as you want.
This way, the next time you need to batch assign users to a new project, select the previously created project at the same tab and click the "Apply" button.

If you're part of a Company you can also share your project with all or some of your teammates selecting them from the Companies tab.

You must be either a project owner or be granted Power User access via the project owner to manage a project’s setting including sharing.

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