Groups are a selection of employees. You can create a group at any time. 5 employees working on the same task? Add them to a group and assign that group to a project manager.
Click the + button to create a group
Enter a name for the group andpress "Add"
Click on the newly created group and search for employees and click on the ones that should be added
Assigning a group to a project leader: Open the "Settings" --> "User" and add the group to his/her user account.