MySQL/PHP: How to do Subscriptions/Joining : <p>In my app a user can create an event and with those events a specified number of people can join.</p>

<p>The best way I can think of is to have columns named something like joined1, joined2, joined3 .. and so on and input the ID number of the user that joined in that column.</p>

<p>My question is how can I input the user ID into a joined spot that is not taken? The number of the joined spot isn't important but having access to the User IDs are. </p>

<p>Thanks for the help!</p>
