This issue might happen if you clicked on the link from email while you are not logged into app.cloudforge.com of the same organization/account. If the invited person clicks the link, and they're already logged in to 'your' organization (The key here is the logged in account should be in the 'same' organization), we just attach permissions to that user for the project(s) they were invited to.
If the users aren't logged in, they'll be asked to create a new account. Because if the person isn't logged in, there's no way for the system to know if they already exist in the organization or not.
This 'new' user sign up form will be presented to the user only for the first time when you are inviting a new member to your organization. Once this user is added and you intend to add same user later to different projects in your organization, he will not be presented with this registration form as long as he is logged in to app.cloudforge.com on 'your' organization.