Would be great if it worked, but it doesn't. Even adding external users to groups doesn't work well. When they finally receive the welcome message, it does not agree with the instructions above; perhaps the instructions are outdated. The instructions say that the guest can email the group, go to Sharepoint, Go to the Shared Notebook, or Remember your group. These are in fact not what the group guest is supposed to be able to do. He/she is supposed to only be able to access Planner. So right off the bat, the whole procedure is messed up.
Signing up for a Microsoft account gives the guest user access immediately to the entire Group and its files, which are highly private. Telling people that they should then go to a specific URL to access the Planner after they have navigated through a confusing sign-up process is just not workable. In any event, when they try that, it still doesn't work. So basically, it was a huge headache that doesn't work, and a waste of time.
Microsoft should be able to do better.