If you want people to be in a 'pending' state until a specific date, you can add people to the Pending tab in the Organization. Then you can make them regular members when you are ready for that to happen. One use case might be those who are going to serve on a committee beginning on a specific date. You can make them Pending members until that date, then move them.
Here's how you change a Pending member to a regular member:
This will move the selected people from the Pending tab to the Members tab.