When you store a contact on the phone, it can be stored either as a Backup Assistant Plus contact or as a Google contact. You can choose to save your contacts as Google contacts by going into the People app, going to Settings, and then going to Contact Storage. If you select Google, all contacts you add from that point on will be saved as Google contacts and will be backed up by your Google account. Note, however, that contacts saved prior to changing that setting will still not be backed up by Google.