The good folks at Domeorama have got all your geodesic needs covered in a thorough and awe-inspiring list of all the different kinds of geodesic hub connectors you can buy:
This is the classic way to connect geodesic struts together. A hole is drilled in the flattened ends then bolted together.To accommodate a drilled hole into your struts, the length of the strut needs to be longer.For example a 36 inch pipe/conduit strut will need to be extended at both ends to drill holes. A rule of thumb is to add 2 X 3/4 inch = 1.5 inches (or about 8cm) more to all struts. That means the center of the holes would need to be drilled 3/4″ from each strut end.
…This is another way to attach geodesic struts together to form a hub. Inexpensive and something you can do yourself, this hub has 5 bolts instead of just one, so very solid. Keep in mind you need to cut your struts accordingly: since connections are vertex to vertex, the total length between centers of the round tube needs to be the length of your strut that was calculated. In other words, your real length will need to be longer because ends are bent.
(via Crib Candy)