<?php if($this->session->userdata('user_id')): ?>
<hr>
<?php echo form_open('/attendees/add/'.$event['id']); ?>
    <input type="hidden" name="user_id" value="<?php echo $_SESSION['user_id']; ?>">
    <input type="submit" value="I'm in!" class="btn btn-success">
</form>
<?php endif; ?>

<?php
class Attendees extends CI_Controller {
    public function add($event_id) {
        // Check login
        if(!$this->session->userdata('logged_in')){
            redirect('users/login');
        }
        $this->form_validation->set_rules('user_id', 'required|callback_check_userid_eventid');
        if($this->form_validation->run() === FALSE){
            $this->session->set_flashdata('attendee_not_added', 'You are already on the list.');
            redirect('home');
        } else {
            $this->attendee_model->add_attendee($event_id);
            // Set message
            $this->session->set_flashdata('attendee_added', 'You have been added to this event.');
            redirect('events');
        }
    }
}

<?php
class Attendee_model extends CI_Model {
    public function __contruct() {

    }

    public function get_attendees($id = FALSE){
        if($id === FALSE) {
            $query = $this->db->get('attendees');
            return $query->result_array();
        }
        $this->db->select('attendees.id, attendees.team, attendees.is_goalie, event.id, user.first_name, user.last_name');
        $this->db->from('attendees');
        $this->db->join('event', 'attendees.event_id = event.id', 'inner');
        $this->db->join('user', 'attendees.user_id = user.id', 'inner');
        $this->db->where('event.id', $id);
        $query = $this->db->get();
        return $query->row_array();
    }

    public function add_attendee($event_id){
        $data = array(
            'event_id' => $event_id,
            'user_id' => $this->session->userdata('user_id')
        );
        return $this->db->insert('attendees', $data);
    }

    // Check attendee exists in event
    public function check_userid_eventid($event_id, $user_id){
        $query = $this->db->get_where('attendees', array('user_id' => $user_id, 'event_id' => $event_id));
        if(empty($query->row_array())){
            return true;
        } else {
            return false;
        }
    }
}