Yes, anyone can leave notes to your designs, even users that aren't members of your team. They just need to authenticate with an e-mail address and their name. However, these users outside of your team can't add revisions or get assets and specs from the design, only members of your team can do that.
In case you're supposed to be a member in a team, but for some reason you're not, you can easily request an invite from the Admin. Just click on Request button like this and we'll automatically send an e-mail to the admin and they can easily confirm your request.