This person is in charge of coordinating the efforts of other organizers, such as making sure that progress is being made.
The key skills of this role are communication and the abilities to motivate, delegate and asserting oneself.
The person to fill this role will usually be the person driving the effort to have an event. Nevertheless, discuss what's involved in the role so that everyone knows what is involved.
It's important that this person resist doing more than coordination (for example, they should not be designing t-shirts or contacting the caterer) as it sets a bad precedent for future events. If the lead's role is too difficult or time consuming, few will volunteer to accept it at future events.
If you see that the Lead is doing too much, you should discourage them from doing so, publicly if necessary.