You can enforce 2FA for all of the users in your account.
To enforce 2FA, go to the gear icon in the top-right corner and choose 'Organisation Settings > Two-Factor Authentication'. Switch the toggle from 'Optional' to 'Enforced'.
This will send a short email to all users in yours account who do not yet have 2FA enabled.
Each of these users will be prompted to setup 2FA the next time they login. They will be able to skip this prompt once, but only once.
Who has turned on 2FA?
To see who has 2FA turned on in their account, as an Administrator, go to 'User Management' and search for the user. The list will display the 2FA status of each user.