You will need to enable access to contacts to add a team member from your address book. To enable contacts, follow these steps:
1. Open 'Settings' on your iPhone
2. Scroll down to 'Privacy'
3. At the top of the 'Privacy' screen, tap 'Contacts'
4. Find 'Abacus' in the list of apps, and make sure the toggle is set to 'on'
Contacts should now display when you add a team member.