There are two types of joints in this structure. The first is where two 10' pieces of PVC have been joined to make a 20' piece. (You can buy 20' PVC that makes this unnecessary, but it's too much trouble for me to transport.) The second is where the top support crosses the arches, keeping them from bending towards each other under the pull of the fabric, and providing support to keep the fabric from sagging much.
To join 2 pieces of 1" PVC that will be bent and under tension, I use a sleeve of 1 1/4" PVC about 7" long. The 1" PVC fits snugly into it, but is easy to get out again. I originally used duct tape (why I hate duct tape) to make sure the sleeve stayed put, but eventually found that the sleeves are held extremely well by the PVC tension. I've never had one budge an inch in the 3 years I've used them without tape. If you're building something that won't be under tension and just need to attach two lengths of PVC, a sleeve with healthy amounts of duct tape works great. To make sure it's centered, I mark the pipes where the ends of the sleeve will go. Then, when it's all together, I check that the two pieces of PVC are butted together in the sleeve and that I can see some of the marks at each end of the sleeve. Simple and crude, but effective.
To join the top support to the arch, I take a piece of innertube and wrap it around the joint, stretching as I go. At the end, I tuck the loose piece under one of the wraps and pull it a few inches through. Very strong, very flexible, and very easy to remove later. This was a suggestion from Danger Ranger years ago, after seeing some pictures of my first structure on the web.
The inner tube pieces are made from old bike tubes, which are available free from friendly bike shops or very cheap at garage sales and such. I take an inner tube, cut off the valve, and cut it in half both ways (length and width). This gives pieces about 3' long and about 2" wide, depending on the type of tube. After you cut them and before you use them, give them a good hard pull to stretch them and make sure that they won't break, as I've seen some really old ones do. If you store them dry, they will last for years, but always test-stretch them before going out to the playa.
I've tried a few things that didn't work, such as using store-bought joints and drilling them for bolts. Even with glue, these sprang under tension. The duct tape method mentioned above worked fine, but was a real pain to get apart.