By default, reviewers can add comments to Review 360 projects without an Articulate ID. This makes it even easier to gather feedback from your stakeholders. They'll just be prompted to enter their email address to post comments so they can receive notifications for replies. (If their email address is tied to an Articulate ID and if they aren't already signed in, they'll be prompted to enter their password.)
If you want to require reviewers to sign in with an account before they post comments, you can do that too. Open your project in Review 360, click the Share tab in the upper right corner of the screen, and uncheck the box to Allow users without Articulate IDs to comment. Then copy the shareable link and send it to your reviewers. If they don't already have Articulate IDs, they can create one here. It's free.