If you have a connected Gmail, Exchange, or Office 365 email account, and add a new contact outside of the software (in your email, Outlook, Apple Mail, etc.), they will be added as a contact in Engage, but it may not happen immediately. Our software communicates with your email, but has to do a "refresh" to pull in the new information (email history, new contacts). That refresh happens each night automatically, but can also be done manually by going to "connected accounts" under Settings and selecting Sync (see screen shot).
Keep in mind that Outlook also has to communicate with your email server and has to refresh its information regularly as well. (Refer.com <------syncs to-----> firstname.lastname@example.org <------syncs to-------> Outlook)