Custom application forms within a project automatically register the people in the form into the project. This is by far the fastest and easiest way to add people to a project, since it lets you consolidate all of the needed fields in one place, in an order which makes the most sense to your organization. See Using Custom Application Forms for more details on how to use custom application forms for data entry.
Custom application forms within a project must have the Delegation and Role fields to be functional.
You can't edit someone who's already in the project using a custom application form, since VSys would try to re-register the person into the project again.