Instructors can offer students the ability to self-organize their own project groups. If this option is available to you as a student, there will be a button at the top of the project page labeled "Manage Groups".
This button launches a modal that allows students to create a group and invite other students. If there is a limit to the number of students it will be shown at the top of this modal.
To invite a student, simply click on the drop down box at the bottom of the modal, select their name and click the invite button. The next time that student visits the project page there will be an invitation at the top of the page they can accept or reject.
All students in a group can submit for a grade. All students on a project share the same grade for the submission.
If you need to remove a group member, see your instructor.