How to add and grant access to external users for an M365 mailbox

JeretC 

Please provide information about how you were able to allow external users sign into a mailbox created in your Microsoft 365 tenant.

From your statement "These external users need to be able to log in to this mailbox to send and receive emails.": the feasible setting is to allow external users send and receive emails with a "mailbox" created in your tenant but they won't be able to sign into the mailbox. The external users will receive emails sent to the email address into their primary inboxes and will be able to send emails to the email address from their mailboxes.

The Microsoft 365 group enables both external and internal users membership but none is able to sign into the mailbox as it does not have a shareable password.

https://learn.microsoft.com/en-us/microsoft-365/admin/create-groups/compare-groups?view=o365-worldwide

Thanks.