Allow Collaborators to join a project without sending an invite by generating a Share URL under your Project Settings.
Open Project Settings by clicking the three dots on the top right of your Project screen. Next, turn Project Sharing to ON, this generates a link that can be shared.
Be sure to Save Settings or the link will not be enabled 👍
Be sure to switch Project Sharing OFF after the Collaborator(s) have joined the Project. If you remove a collaborator, they can rejoin the project as long as they have the URL to join and Sharing is still set to ON.