To be able to reply to a comment, first you need a Modelo account and to be invited into the project as a collaborator. Sign up is free, please click here.
Once you are signed up, ask your collaborator to invite you into the project. Check the resource below on how to do that:
Once you are added into your project, you will be able to see all the comments on this project, you can reply to a comment by simply clicking on "Reply" and start typing away.
You can notify different team members by assigning the comment to them.