Choose your Settings and click Security in the left menu.
- Click the green button Enable two-factor-authentication.
- Choose if you want to activate the authentication via SMS or Google Authenticator.
From now on you will need both to log in, your password and your phone: Grape will ask for the code that you'll get via text or the mobile app when you (or anyone else) tries to sign in to your account from other computers.
For admins: How to get an overview about your members and their 2FA is explained here.