I made the plunge...

After riding for months and really enjoying it I finally stepped up and bought a bike. It's a nice bike that should get me beyond a few years (Canondale Synapse Carbon - disc with ultegra), and was a great price. I have picked up the safety gear, tools etc. Read through countless articles. Have simulated tire changes at home and, last but not least, tipped over when learning to deal with cleats.

That being said...
How to you migrate into getting into a group? Everyone is so fast and I still need to lose some weight. Do you just go a part of the way, or go solo for awhile until you have some endurance?

Other than the rules of the road, are there any courtesies I should be aware of?

Well, you have learned one thing. Buying and expensive, lightweight bike does not instantly turn you into a fast road rider. No matter what, it should be more pleasurable than riding an entry level bike.

It takes time and effort to build up endurance and increase your speed. Google "local bike clubs" for your particular location and visit their websites. Look for one that separates their rides into categories by riding speed and then start with one of the more sedate groups. If you pick the fastest group ride you are going to be pushed beyond your ability and likely be left behind. Frustration like that take all the joy out of riding. Once you get better at it, move to the faster groups and enjoy riding.

I would suggest googling riding groups in your area. A lot of groups have different levels of rides for people of different levels to get in with. Definitely need to do some riding to get into better physical shape and acquire the skills necessary to ride with groups. Most of my riding, at least 95%, is solo. I have been doing it for a lot of years. I do have a couple of guys that I ride with sometimes, and I also, at times, hook up with others I meet on the road. Getting into the maintenance aspects of being a bicyclist can really add to the depth of your involvement.

If you bought your Synapse from a local bike shop, I would ask them if they know of any local rides or clubs that are suitable for newbies. Keep in mind that the term "group rides" comprises a huge spectrum of rides from super-mellow rides from pub to pub, to club rides with cue sheets and published speeds, to the Tuesday Night Worlds which are essentially unsanctioned all-comers road races.

You will probably want to start with something where there is a ride leader, an agreed upon route and pace (with or without cue sheets), and a no-drop policy. When you go to your first group ride, let the ride leader (if there is one) know you're new. Hang around the back to get used to feeling the draft and don't worry about taking a pull.

Beyond the rules of the road, the universal etiquette of group rides means showing up self-sufficient. Meaning have your own means of fixing a flat or minor mechanical, and water and food if you need it. Call out hazards, particularly potholes or bad pavement. Call out cars passing.

Nice bike. That one was on my short list when shopping for a new bike this spring.

A right of passage. Congrats!

As suggested, look for local bike clubs as well as bike shops. Many offer rides for different levels.

My LBS has a weekly "beginner" group ride of 12-15 miles, leisurely pace, no drop, along with a brief intro to group ride etiquette.

Another group ride has A-B-C groups ranging 30-60 miles, 16mph to 22 mph pace. Something for everyone.