Under Home => Tasks -> Tasks, you may click on the “Add a task to this project” button and then select the appropriate project.
You may also add a task to a currently open project by clicking on the right edge of the project tab in the top frame and then selecting “Add a task” in the pulldown menu.
In the “New Task” page, you may assign users as “Assigned” or “Reviewer” in the task assignment grid located in the “Assignment” section. You may refer to this help page for information on the differences between an “Assigned” user and a “Reviewer”.
Unless you check the “Do not notify” checkbox prior to clicking on the Save button, the user(s) you assign on the task should automatically receive an e-mail notification. Task assignment/update e-mail notifications can be turned on/off in user profiles. By default, when users are added in the account, all automatic e-mail notifications are enabled.