In the project admin (Manage -> Projects) you can also (un)assign users. Select an existing project, below the edit project panel there's a panel with Project assignments. This panel lists the currently assigned users to this project and also a "new assignments" button.
The "new assignments" panel is split it up in two lists, the list on the left with "All users" is the list of all available active users in eHour. From this list you can select multiple users, the selected users are listed on the right below the "Users to assign" title. To deselect a user, click on the name in the "Users to assign" section.
Select the users you want to assign, fill out the form and click "save". An individual assignment will be created per user with the properties specified in the form. Have a look at the image on the right of this text, for this selection 4 users will be assigned and each user is allowed to book 300 hours.