How to add all users instead of one by one from the PRs?

How do I add all users? This screen shows that they will be displayed here, as they commit and add pull requests.

But so far, I haven’t seen any suggestions.

The only suggestions I’ve seen is when you open a pull request, there is a notification at the top with a link to add the user. But we have 20+ users, I can’t be bothered to add every user manually like this. Is there any automated way of adding all users?

Also, why only “owners” can add users? I expect “admins” to have the same right but they do not.