When someone requests to join Confluence, the admin interface shows what they're requesting access to (e.g. "a page on the RSI space"), If you want to grant them access as a guest, you first grant them access then assign a space. The space lookup doesn't search by key though, so unless you recognize the key and know the name of the space, you have to exit the process to find the key. The interface should make it easy to assign a guest to the space where they made their request. Some ways to make it easier would be to:
I'm sure there are other options. This isn't a bug and you can't submit suggestions directly to Atlassian, so I guess this is a discussion.