Microsoft Teams uses modern authentication to keep your sign-in experience simple and secure.
Modern authentication is a process that lets Teams know that you've already entered your credentials (like your work email and password) elsewhere, and you shouldn't be required to enter them again in order to start the app. The experience will vary depending on a couple factors, like if you're working in Windows or on a Mac. It will also vary depending on whether your company has enabled single-factor authentication or multi-factor authentication (multi-factor authentication usually involves verifying your credentials via your phone, providing a unique code, entering a PIN, or presenting your thumbprint). Here's a rundown of each modern authentication scenario:
If you're already signed in to other Office apps through your Office 365 Enterprise account, when you start Teams you're taken straight to the app; there's no need to enter your credentials.
If you're not signed in to your Office 365 Enterprise account anywhere else, when you start Teams, you're asked to provide either single- or multi-factor authentication (SFA or MFA), depending on what your company has decided they'd like the process to entail.
If you're signed in to a domain-joined machine, when you start Teams, you might be asked to go through one more authentication step, depending on whether your company opted to require MFA or if your machine already requires MFA to sign in. If your machine already requires MFA to sign in, when you open up Teams, the app automatically starts.
When you start Teams, your machine won't be able to pull your credentials from your Office 365 Enterprise account or any of your other Office applications. Instead, you'll see a prompt asking you for SFA or MFA (depending on your company's settings). Once you enter your credentials, you won't be required to provide them again; from that point on, Teams automatically starts whenever you're working on the same machine.
There's no way to switch user accounts once you've completed modern authentication if you're using a domain-joined computer (for example, if your tenant has enabled Kerberos). If you're not working on a domain-joined computer, you can switch accounts.
To sign out of Teams, click your profile picture at the top of the app, then select Sign out. You can also right-click the app icon in your taskbar then select Log out. Once you sign out of Teams, you need to enter your credentials again to launch the app.
Modern authentication is available for every organization that uses Teams, so if you're not able to complete the process, there might be something wrong with your domain or your company's Office 365 Enterprise account. Your best bet is to get in touch with your IT admin for more information and next steps.
For more information, see Why am I having trouble signing in to Microsoft Teams?