Imagine this scenario: You're a development manager in a small organization. You have a product and a couple of customers. Over the course of the next year or so, you add a few more developers and a few more customers. And, your customers want to know when you'll improve the quality of the product, because they're finding more defects than you are. Time to hire some testers! I see this strategy needed most often in small organizations, but larger organizations are not immune from having a too-homogeneous group.
Cultural fit is key here. As you interview candidates, make sure their values are similar to the existing group. The longer the current group has worked together the more difficult it will be to integrate new people, so making sure their values mesh is the most important thing.So what about functional skills–aren't they important? Yes, after cultural fit. The abilities of each person–especially in a small organization is key. If you have great developers, you better hire great testers, or the testers won't be able to pull their own weight.
Once the entire output of the group has to change, focus on cultural fit to make sure the new people can fit into the group.