Go to your contacts page via the bottom left button on the home screen navigation bar.
On the contacts screen, you will see the list of people in your workspace and the option to invite a new person to your workspace.
Click 'Invite Member', which will open up your phone contact list. Once you select a contact an SMS message will be created inviting them to get the App and sign in. To complete the invitation process send the SMS.
As you add team members you will see them listed on the top of the home screen.