It looks like standard two factor authentication is not available for community users. Does anybody know a work-around for this?
Two Factor Authentication is supported for Community Users as pointed out by Rob Cowell.
For standard user license type, you my be accustomed to enabling 2FA via the System Permission
Two-Factor Authentication for API Logins and
Two-Factor Authentication for User Interface Logins. However, you might not see these options for community profiles.
Per Set Two-Factor Authentication Login Requirements and Custom Policies for Single Sign-On, Social Sign-On, and Communities, you need to modify the community profile’s Session Settings and set
Session security level required at login to be High Assurance.
When user attempts to login to community with their
password then they will automatically be prompted to provide 2FA. Here’s screen shot of a community user being prompted to setup Salesforce Authenticator app for the first time: