There is a subtle or perhaps not so subtle distinction between "hiring staff / adding people" and "building a team".
I'm sure this doesn't just apply to software development teams but a broad range of areas. I guess you can just get a bunch of brilliant people and throw them together and hope for the best. You might get lucky. What I'm about to say I think many people should get if they have experienced working in a few different teams, so this blog is just for those that aren't quite up to speed with the distinction between "building a team" and "adding some new staff".
If you think about winning sports teams, it's not entirely a sum of the talent of each individual of the team that determines the result (in team oriented sports), but rather the way they work together as a team combined with what talents they bring in combination. In all A-league teams, I'm not saying the players can be B or C grade players and the team can win, the players do need to be top class, but might not all be the best within that class if they work harmoniously with the group as a whole to bring about a win.
The criteria for hiring someone should include their ability to do the job they are hired to do. That much should be obvious. But when selecting between candidates that can fulfill the basic requirements, I believe one should look to the candidate's ability to blend and complement the existing team and to compare that between the candidates. (hopefully with any luck you will have more than one candidate that fulfills the basic requirements of doing the job).
So how to test or evaluate the ability for the candidate to blend? Simply invite them to join in on a team activity. If the team has lunch together, invite them along to lunch. Ask them to sit in on a team meeting, or perhaps on Friday evenings the team gather for snacks and chatting before heading home, perhaps time the interview at a time to precede such an activity and ask if they mind staying for that if you feel the candidate is someone potentially worth hiring.
The idea is to disarm the candidate to feel relaxed enough to reveal more of the real them. Interviews are usually quite artificial situations. The idea is to step out from that and observe the way the candidate interacts with others in a more natural/genuine environment/situation. Then it will be possible to also ask the existing team what they think about the potential new person at a later time.
I can hear some saying/thinking "but we don't have any activity like that, what do we do, everyone has lunch separately and goes home early on a Friday evening". To which I would say that it doesn't sound like much of a team environment. Sounds like there are bigger problems to resolve before adding more people in to the mix. Perhaps adding staff is trying to solve the problem of poor efficiency with more hands on deck however often adding people causes initial inefficiencies and diminishing returns as the number of interacting staff increases. New trainees drag down the existing team with questions and effort getting them up to speed. Adjusting to a new team dynamic which is upset with the changing of staff is another factor. And there is a diminishing return as the amount of communication exponentially increases with the numbers of staff interacting.
Just to simply make the point if it is not clear, if it wasn't so, any project with say 5 people that takes 12 months if there was no diminishing return on more people on the project would take 10 people 6 months, 20 people 3 months, 40 people 1.5 months, 120 people 2 weeks, 240 people 1 week, 1280 people 1 day, etc. Unfortunately reality doesn't work that way, communication and coordinating over 1000 people to come together and do some task on a given day and divide the task up so the 1000 people don't spend as much time communicating with the 999 other people to ensure they do the right 0.1% of the task will take more than a day. Probably much more than a day just figuring all that out. And that's assuming a task that can be divided in to 1000 non-dependent little bits which is rarely if ever the case.
Building a team also involves getting the team to do stuff together. Perhaps in some work environments it is not necessary to have a team, it really is just a set of individual efforts. I can't really think of any real concrete examples where a company is formed and contains a set of non-cooperating individuals. It's kind of not really a company, if everyone's job is unrelated to each others, then surely they don't require being a member of a company and could each operate as individuals, eg as sole proprietors. However that sounds rather lonely. Having positive social interactions is an important aspect of a job and working with others and getting stuff done.
So what it comes down to is getting stuff done, and getting it done efficiently. When the team is working well together like a well oiled machine, stuff gets done, it's enjoyable and a positive environment. When there isn't any cohesion in the team, no one gets along, interactions are negative, it greatly impacts on getting stuff done. That is the bottom line.
Usually the idea of adding more staff is to get more stuff done. If you are in agreement with that statement then I hope I can finish here for you. Sometimes it's to build an empire for the aggrandizement of someone with ego issues in which case I guess the idea of "building a team" must seem like a strange concept when the idea of "adding staff" fulfills faster the aim of building an empire. If this is you, keep reading. Even empires can crumble, particularly when there is rebellion. You still need relatively happy workers to build an empire. I do think most people prefer being in situations with positive social interactions over negative ones or over having a lack of human interaction for prolonged periods so I think most people want to belong to a productive working team, rather than simply a minion for the aggrandizement of an ego maniac running around pretending to look busy. Even if empire building, you need to create a cohesive group, infuse culture, create positive attitudes, promote a rewarding and productive environment all to keep the empire from rebellion.