When training for an endurance sport such as triathlon, are you best to join a group for your training sessions, or would you be better off doing the majority of your training alone? That's actually a difficult question with a lot of variables. Group training provides a lot of benefits: camaraderie; people pushing you when motivation is low; the social aspect. On the other hand, when taking part in a group setting you're likely not doing what you specifically need to do... strengths & weaknesses of each athlete are a very individual thing.
Solo training provides so many other benefits that being part of a group doesn't provide. As mentioned above, to be the best athlete you can be, you must do specific workouts designed around what will bring further development of your fitness levels. When we have kids to run after, work that needs to be done, homes that need to be maintained... this all takes away from one of our most precious commodities: time. Therefore, if you want to be the best athlete you can be you must make EVERY workout count!
We need to remember how to feel. That may sound ridiculous, but it's true. We're inundated with analyzing numbers, what others are doing in their training, the latest equipment on the market... and we lose the sensation of how the body feels. When training on your own you can feel what good form is without other distractions. The body will do what the mind tells it to, we just need to figure out how to do that. And training alone is one way.
Training alone will also help you build mental stamina. When you're 70km away from home on a long bike ride & feeling tired or broken there is nobody else out there to help you get back home. It's entirely up to you. But it'll toughen you up. I'm willing to bet that those who give themselves an exit strategy in training are the ones who walk or quit in a race... so develop that toughness on your own!
In summary, I'm not saying that group training is a bad thing. If your sport is a big part of your social life then take part in it. But make sure to mix in a lot of solo training sessions, especially on the longer distances when your mental toughness is further challenged.