MicrosoftNice write up Toby Bianchi, thank you. I came here looking for more details about inviting external users to Modern Team Sites/Office 365 Groups. This would still be helpful to add as you mentioned in an above post. Also, I would like to echo the comments of @Carry4IT Megens above regarding users who are already logged into a Microsoft account on a browser when the click the email invite. There is no graceful way to allow a user to select which Microsoft account they want to link to the Office 365 Group or create a new one, and often they don't even know which account they enrolled with if their passwords are the same between multiple accounts. This presents a problem when the user needs to log in fresh, they don't know their account and you can't help them reset the password to access your site. Using a private browser session seem to be the only way to cleanly do this, but communicating this to the external user you are inviting is difficult because the invite email is generic. It am sad to see this issue carry over from the classic experience, hoping it can be addressed.
Suggestion for how it could work: when the external user clicks the invite link they are presented with all the possible Microsoft accounts used on that browser (like how you can switch between gmail accounts) to enroll with, and an option to create a brand new account. This way a user could decide if they want to use their work account, personal account, or neither and create a new one.
The features and benefits of the guest access are amazing once you get past the registration part, but the way invites are currently architected it only works for small teams that you can manage through the invite process. I currently dread the requests for a new Team Site that needs to have 20+ external users to share files...
