Every project within Fieldlens has its own directory of the companies and users on that project. Access that by clicking People across the top of the web app or the bottom of the mobile apps.
Fieldlens users maintain their own profiles and can include their phone number, email address and other contact information. Each registered company will also have a company administrator that will maintain a company profile.
If you've added a person to your project that does not have a Fieldlens account you will be able to add in their contact and company details from the People page on the web.