Is a 29.5 km/h average speed decent for a 90 km ride (on road) with an elevation of 1200 m? I have been riding about 150&nbsp;km a week commuting and on the weekend. I frequently do rides over 80&nbsp;km. I'm riding a carbon cyclocross (Ridkey x-fire) with slicks.

My latest ride was on the Central Coast, New South Wales, in a very hilly area. I rode 90&nbsp;km with an elevation of 1200&nbsp;m with an average of 29.5&nbsp;km/h. Would I be able to join a group ride and not get dropped?

Will switching to a dedicated road bike make much difference to my average speed? I feel like my setup is good enough, and I frequently pass other riders on their high end roadies.
Riding at 30kph average for 3 hours, in a hilly area is a solid effort.  Assuming your pack riding skills are sufficient, you will also likely do fine in in a club ride that averages 30-40kph (but see the pack riding primer below). Club rides will have a faster pace than what you are riding now, but you will also be working a lot less (about 30% less) at any given speed because of drafting.

Now of course this assumes your listed average speed is an honest average speed. That is you started your timer, rode 90 km, then stopped the timer and found you averaged 29.5 kph.  If by "average" you mean when you took the time to look at your speedometer you were often doing 29.5kph, then it is harder to assess.

Short primer on pack riding.

It is hard to give a definitive whether or not you will be dropped because solo riding and pack riding are very different disciplines and club rides also come in many different forms, from very competitive "A" group training rides with a drop policy (i.e., you are essentially on your own if you get tired or get a flat - very advanced groups), to others focused on sustained pace (what you likely do now) with no-drop policies.  If you join a no-drop ride, then the answer should be "no, you will not be dropped."

Riding in a "fast" group

Because you explicitly asked about getting dropped I will assume you are looking at joining one of the faster club rides that may have a drop policy.  In this case your pack riding skills will be just as important as your base fitness.  If you feel comfortable in a group you should have no problem holding tight to a wheel, where you can  will get a good draft and will be able to sustain higher speeds.  Riders that feel less comfortable will often allow big gaps to form or ride off the back to feel "safer."  This will require a lot more energy for a given speed.  You will also be able to read the pack, and keep a good drafting position as the group surges.  If you get taken by surprised in surges you can find yourself dropped quickly.

Assuming your pack handling skills are sufficient, riding in a fast group will also require two types of fitness, endurance fitness and peak output.  The endurance fitness is what you are doing right now and will help ensure you can put in an effort for 3-4 hour duration of the club ride. The peak output fitness will be important to handle pack surges.  Often competitive club rides will have sections where there is either an all out sprint or it is "common" knowledge the next x kilometres will be all out effort.

If all your milage has been at one consistent speed, then you might find these types of surges difficult to handle, even though your base endurance fitness is excellent.  If you find this happening, it isn't necessarily because you lack fitness, but you may simply not have all the right fitness components. Supplementing your distance training with interval training can go a long way. 

Many novice riders make an number of tactical errors that make it harder to finish the ride.  A common mistake is try to take long, long, pulls to "prove" how strong they are.  Keep you pulls short (i.e., under a minute) to ensure you have lots of energy and don't build-up a lactate debt.  As you do more club rides you can start trying longer pulls if that is your thing.  Another common mistake is putting in too much effort, too early.  Many fast rides will do the hard efforts on out-and-back sections.  This means you can get temporarily dropped and catch back in as the group comes back from a hard effort. If you are unsure if you can sustain repeated high intensity intervals, it may be best to opt out of effort and catch back in after everyone has finished tearing each others legs off.  You would be surprised how refreshed you can feel by skipping an effort or two. 

If you feel comfortable in a group, have no problem hold tight to a wheel and can sustain repeated bursts of speed, then you there is a good chance you that you can hold in on an advanced group ride. 

Other Types of Group Rides

As eluded to earlier (and in other answers) not all club rides are created equal.  Some rides are focused on building base fitness and camaraderie.  These groups are usually focused on riding a consistent pace (no surges) and usually have a "no drop" policy. These types of rides are a great if you are new to pack riding. 

Many clubs will also have multiple different rides on any given day so you can do some trial and error to find the one that best suits you.

I suggest chatting up your local clubs, ask about their club ride policies and see which one best jives with what you want to do.