In Manage -> Assignments you can also assign a user to multiple projects at once. Do the same as assigning a user to a single project but instead of one, select multiple projects in the dropdown. This will create a single assignment per selected project with the specified properties.
In the case shown in the image on the left this will assign the user to 3 projects, Galileo, Gemini and Glory. The user will be teamlead with a rate of 95 euros on each project and is allowed to book 320 hours on each project. The latter is important to understand, the created assignments will not share the allotted time but the allotted time counts individually per project.