Only someone with admin rights to the repository can add collaborators, so there's no way for a potential collaborator to automatically add themselves via a URL.
However, you can use the GitHub API to programmatically add collaborators. You can make use of the API in several ways. Here are two possible approaches:
- Write a script to add any and all users as collaborators, either as a one-time execution or to use on-demand when you have new users to add.
- If you have access to a web server, Microsoft form, or some other method for having the user submit a request or respond to a request, write a server-side script to add any user who responds to your email/form/etc. as a collaborator.
The second approach would be more precise, as it wouldn't add users who don't want to be collaborators, but it would also require more setup.