For newly added or for existing employee if you want to include him to all project & task you can do it from the Employee Page. 

Click on Projects from the MoreDropdown as highlighted in the above screenshot, you wil be directed to a page of Project allocation List, Uncheck the button Show Assigned Projects, then all projects in the account will be displayed. 

Select Check All Option as shown in below screen & click on update at the bottom end. Please refer below screenshot. 

Once you done with this process click on Task from the Moredropdown of Employee page to add all task to the employee. Please refer below screenshot. 

Please follow the same steps of assigning what actually done in project allocation page for assigning project task.