Allow inviting collaborators by email or your product is essentially useless for collaboration for our needs.
Look at how Google Docs does it.
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EMPLOYEE0It's definitely a planned feature. We want to make the whole experience of adding a new collaborator much smoother.
For now, you can go to Settings > Invite and invite a new user to Bundlr by email. You should be notified when that user signs up, and then you can add him/her as a collaborator of your bundle. (Definitely not the best experience, but it's currently the best way to do it.)
Thank you for your feedback.