Most common cause of the symptom
The user you are searching for is in Private Mode. If the user you are searching for definitely exists then it is likely that they are in Private Mode and they need to go to their settings and turn this off to become discoverable.
Other cause of the symptom
The user does not yet exist.
We allow Insite Teams users to activate Private Mode to make themselves undiscoverable to other users when searching to add them to a Project or Plan. This is an optional setting that can be used to protect our Users' data as other users can are able to find a user's name, position, company name, and profile image (if these are set) by searching with their email address.
1. Ask the user who you are searching for to check if they are in Private Mode.
2. Double check that you are searching for the correct email address and make sure that the user you are looking for is definitely signed up and has verified their email address.