To register more than one ecobee to your existing account, first login to your ecobee account from the Web Portal or the mobile app.
Registering an additional thermostat via the online Web Portal:
1) Click the 
icon in the top right corner of the screen
2) Click ADD THERMOSTAT
3) Enter the 4-digit registration code from your thermostat and then click NEXT
4) Proceed through the rest of the registration process by filling in the required fields
5) Once you’re finished, return to your thermostat and tap OK at the bottom right corner to complete registration
Registering an additional thermostat via the app:
1) Tap the icon in the top left corner of the screen. If this icon is not here, tap the green arrow in the top left corner instead to back up a screen
2) Select ADD/INSTALL DEVICE
3) Tap REGISTER YOUR DEVICE
4) Tap "ecobee Thermostat"
5) Enter the 4-digit registration code from your thermostat and then tap NEXT
6) Proceed through the rest of the registration process by filling in the required fields
7) Once you’re finished, return to the thermostat and tap OK at the bottom right corner to complete registration