Batch import via CSV is now implemented.

See How do I add multiple users to a project at one time? for details.

(Education accounts should view this document instead: Inviting students to a class/project)

Users on paid plans can duplicate projects, optionally including users. So if you're creating projects with the same set of users frequently, you can set up a project as a template and duplicate the template instead of adding a new project manually. For existing projects it's probably best to set up a CSV with the standard set of users and roles and import them.

Good news, this is in progress. We'll be able to share more soon.