Shhh... Plan in secrecy.
In the upcoming 5.0 release (coming within weeks), we have developed a new project state called "Draft" mode. This new feature will allow you to create a new project and assign people without giving them visibility. Here are the draft project visibility and notification rules:
People assigned to your draft project will not see the draft project until it's activated.
They will not receive any email invites until it's activated.
For those you want to have visibility to your draft project, such as helping you with planning, assign them to the Project Administrator role and they will have access.
Stay tuned!