Along with being credible and trustworthy, you can get results without direct authority by using your influence. Effective influence skills are probably the biggest differentiator when getting results without authority. When you influence, you engage and enlist others in getting results – without necessarily having formal power. Influence is not about forcing or exerting – it's about getting people on your side for good reasons. Three ways to promote influence are through language/effective communication, reciprocity/exchanging 'currencies', and persevering through resistance.
Anyone who wants to develop or refine their skills for getting results using personal power.