When someone invites you to their Sharepoint site, you'll get an email similar to the one below. There's a good chance that it'll end up in your SPAM or junk folder. You should never accept an invite that you're not expecting. Assuming that you are, click on the link in the email.
If you already have a personal or business with Microsoft under the address you were invited at, choose the appropriate. Otherwise, choose the option at the bottom to create a new account. You can read more about the process of creating a new account in this tech article.
Assuming you already have an account, you should then be able to log in normally using your existing Microsoft account login and password.
Because we enforce 2-factor authentication, you may be asked for additional information however. If you prefer to receive a text message or phone call with your authentication code, choose "authentication phone." If you prefer to use an authenticator app like Microsoft Authenticator or 1Password, choose "Mobile App." For the purposes of this tutorial, we'll assuming you're using your phone. To view a video on how to setup Microsoft Authenticator, see this video: How to register for Azure Multi-Factor Authentication - YouTube
Fill out the information appropriately and choose next.
Enter the verification code you received via text or phone and click "verify."
If all goes well, you'll receive a message that you were successfully verified. Click Done.
After you've verified, you will likely be asked to log back in once more, this time sending a verification code as well. Thereafter, you should be logged into Sharepoint site.