You don’t have an email that is generated by Microsoft and were invited to a sharepoint space? You can still continue.
Click on the link provided by the user
Enter the email address that was used to received the invitation and click on next
The system will send you a second mail to confirm it’s you. Type the code in the next screen and click “sign in”
You are now logged with your email account for 24 hours (unless you are in inprivate or incognito mode or clean your cache)