To search for a user click the People Picker button (looks like an open book) (Figure 6).
Figure 6) Select Users
Then type the user's email address and hit the Enter key or click the magnifying glass icon.
Note that two results were returned (Figure 7) for a search on "firstname.lastname@example.org", one for "Live eMail: Shetab Live Users" and one for "Dewberry ADFS". Select the "Dewberry ADFS" user to add a Dewberry user. Note: ONLY use Dewberry ADFS for Dewberry staff, DO NOT use this provider for external email addresses - it will not work.
Once you've found the user you are looking for, select their account on the right side of the people picker and click Add ->
Figure 7) People Picker search