It's even easier if they are running the event. To join up, just ask the person selling the maps or giving you your start time. There is usually a person in charge of new members. They will give you all the information you need. If you do join a club then it's a good idea to get to know other members of the club.
Many clubs hold regular social evenings or training sessions.
This is a very good way of meeting people in the club. By far, the best way of meeting other club members is to help at club run events. There is a Network Newsgroup called 'Onet' dedicated to orienteering internationally. You can get more information on it in the orienteering links page . The Irish Orienteer is published every two months and is the bible of Irish Orienteering.