Instead of removing the items from the array with splice(), why not just set the value to null or undefined?

Then when you're adding a new user, you can just scan through the array to find the first available slot.

javascript arrays are simply lists of items - they're not keyed to a specific key like you might be familiar with in PHP. So if you want to keep the same position in the array, you can't remove other items - you need to keep them, and just mark them as empty.

You might scan through something like this:

var users = [];

function addUser(user) {

    var id = users.indexOf(null);

    if (id > -1) {

        // found an empty slot - use that

        users[id] = user;

        return id;

    } else {

        // no empty slots found, add to the end and return the index

        users.push(user);

        return users.length - 1;

    }

}

function removeUser(id) {

    users[id] = null;

}