After for example a data import (see How to use CSV format related contacts and companies import?), if you want to assign newly imported contacts and companies to a user-project, follow the procedure below.
1) Go to the Query module (top menu -> Query)
2) Click on the center of interest corresponding to the import (for example: "Import 2015-08-05 15:22:45").
Click on "Search".
3) You get the list of imported contacts and companies.
Click on the button "Add to a project".
4) A form appears, allowing you to choose the project with which to associate the contacts and research companies.
Choose the project and click the "Add to Project" button.
The contacts and companies are now linked to the project you have specified.
Project users' rights take into account imported elements.