While I'm sure there are plenty of "Lone Wolf" types our there, training partners are the unsung heroes of our training. Be it for triathlon, running, cycling or anything, these are the people that help get you where you need to be for almost no cost except friendship and reciprocated support. The one "rub" here is that it may take some time to find the right people who fit with your training philosophy.
Friends are always great to try out first for this role, but their thinking on what a training session should accomplish is not always the same. This isn't a bad thing, just not what a training partner is for. Heading out on an intense hill climbing expedition with intervals may be your objective, your friend may just treat cycling as their social interaction for the day and dilly dally through the ride. Not a great combination and ends with frustration either for taking too long for an expected ride, or not attaining the effort needed.
Searching outside of normal friendships is another step to take either through a training group or regular group rides. These have a wide range of abilities and philosophies to take into account. Putting a staunch Zone 2 athlete together with a minimalist high intensity athlete is a recipe for disaster in the long run. Not saying either way is better, at least not in this posting, but to each their own, and training philosophies should be respected. Once you find those with the same attitude towards training you'll find these become good friends in the end as well.
Skill is an elusive aspect to account for, especially with triathletes. We were all once really good at one sport, and trying to catch up in the others. Nobody can be perfect in all three, so some understanding is necessary when training with someone in the weak link. Mine is swimming for the sake of this post. I get lapped in the pool all the time. While I do have the skills on the bike, I'm a good 20 pounds heavier than most of my training partners, so I'm last up the big hills, but not by much. The real skills to be aware of are etiquette and proper pacing. We covered Pacelining on my Coach's blog last month. Other things to keep in mind is people's self sufficiency. While training partners should and will always help each other out, with flats or injuries, there are instances where people get lost, or you just lose them. I'll be the first to go looking for them, but knowing they will either be on their way, or trying to fix their problem on their own is much better than finding them stranded and having given up. Riding with someone who is oblivious is just the worst as well. I can take accidents happening, but not when someone rides into the road after a pull without looking.
All in all, when you find those people you can train with, you'll bend your workouts to fit theirs as well because the end result will be better and more enjoyable. Adding some distance here, or pushing intensity there is easier with others around. Climbing Tunitas this past weekend, every switchback I would see my buddy Mike. I was pressing to close the gap the entire way up to as a result. I love my toys, Heart Rate and Power Meter, but going outside those tools it was much more exhilarating turning the effort into a mini race and chasing someone.