I'd like to send out a URL to an external team such that anyone who clicks on it can join my private repository as a collaborator using a read-only role. I don't want to individually add every person who needs access, nor do I want to give someone else the authority to do this.Is there any way to do this using Github? 
Can I allow collaborators to add themselves on Github?
github
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.