You are going to love and HATE my answer.
The part you'll love:
If you want a heart rate monitor, get the simplest, cheapest one you can find. Make sure you can get replacement chest straps. Make sure the numbers are big enough to see. All you need is a number.
Now the part you'll hate
1. Consider mounting it to your bars. You need to see it. If you are in a paceline, you do not need to be twisting your arm trying to read the darn thing. You want the info, like your tachometer, right in (or as close as possible) your line of sight. When you are aneorobic, this is huge.
2. You have to figure out how to use that number to make you more effective, which unfortunately means realizing that the HRM is not a measurement of work, it is a measurement of how your body is reacting to work. To measure the actual work, you'll need (the part you'll really hate!) a powermeter. That way you know how much work you are doing, and how your body is responding to it. Powermeters used to be super expensive, some are getting cheaper now.
3. Group rides are all over the place. If you are tagging off the back, you will continue to tag off the back (hrm or not) until you modify your training routine. Don't get me wrong, they are a great way to get your hr up and get motivated and talk trash- I love em'. With a HRM, you will simply be able to quantify your misery. If you go for the hrm, train alone at least some of the time, on a trainer, holding a set heart rate over a set amount of time. Google interval training for cycling to get endless training suggestions. Find some that you like- use them. You want to get used to figuring out how long you can hold a certain amount of power for a certain amount of time, and how your hr responds. It is the personal training that will make you stronger. The group rides are great for fun and cardio aneorobic training, but to progress, you need a structured plan that targets your own threshold level, not the groups. To optimize that, a HR helps, but a powermeter is what really sets you up on a path to huge improvement. Without a powermeter, you are just peddling hard and see your heart rate as a delayed (about 15 seconds or more) response to what you were just doing. with a powermeter, you know exactly where you are at, how much you've improved, and how your body is responding through percieved exertion and heart rate.
A very slippery slope indeed! I'm not trying to talk you into a powermeter- enjoy the heart rate monitor first- But as far as the HRM, get the simplest one you can find. No bells whistles, or gimmicks. Even if you eventually get the powermeter, the hrm is just that- a number. If you want to smash your friends at the local ride, go undercover, learn different training routines that will build muscle and cardio, use the HR monitor understanding it is just a reaction to that, and then when you go on the group rides you can smash em' (at any age).
Oh yeah... regardless of hrm you get- also have a stopwatch on your bars if you want to get faster, then you'll know how to measure time as well.
There you have it power+time=heart rate (how your body responds to the first two). This info lets you fine tune your interval training for max improvement.
I just cluttered up your bars bigtime! sorry
Good luck.
Ron