Taken from page 125 of the 2008 "Navigation System Supplement (2nd Printing)":

Adding a new phone to SYNC
Note: This is a speed dependent feature. For your safety, this procedure cannot be completed if the vehicle is traveling at speeds faster than 3 mph (5 km/h).
Note: SYNC can store up to 12 previously paired phones.
This feature allows you to add (pair) a new Bluetooth enabled phone to
SYNC. To access:

1. Press the PHONE hard button on the navigation system.
2. Select the ‘Settings’ tab on the touchscreen.
3. Select ‘Add’.
4. When prompted on your phone’s display, enter the PIN provided by SYNC.

Depending on your phone’s capability, you may be prompted with additional options from your phone or SYNC such as downloading the phone book/call history and setting a favorite.