Types of Organizations
There are all types of organizations, and the type will affect how a particular group approaches its goals and how you can exercise your skills and learn more about leading.
Ad Hoc or Standing
Latin for “to that,” the ad hoc group is one that exists only so long as it works on a specific task. There is an intended finite lifespan. The positive aspect is that with such a focused existence, you should have an easier time concentrating on the team's goals. However, there will be less infrastructure and organization to lend a hand in accomplishing things. The majority of ad hoc groups seem focused on politics and social change. Depending on your interests and leanings, you might find that standing organizations and their goals are a better match.
Because of the ephemeral nature, any hierarchy is likely to be at least somewhat disorganized and malleable. Under the circumstances, the best approach to leading is probably using the techniques of leading from within. Another consequence is that your leadership opportunities will likely be short-lived, and there won't be an ongoing entity that could easily provide a reference.
A standing organization is one that has continued existence. Some of the strengths are actually the inverse of the ad hoc group. By being around over time, they have the opportunity to develop more robust structures to support their goals and provide help to you and the team you have or build. There is a hierarchy, and so you might have opportunities for positions that might add to authority and ease some of the issues of dealing with volunteers.
Sometimes an ad hoc group will turn into a standing organization if it achieves its initial aim. The classic example is the March of Dimes, originally organized to raise money for polio research. It found itself out of a job with the advent of the Salk vaccine, so management reinvented it to address the problem of birth defects.
Professional Versus Volunteer
Many nonprofits have professional staff and volunteers. The paid workers are necessary to maintain continuity and to provide the amount of time that would be impractical for virtually any volunteer. People can donate time while still working for a living. However, the paid staff will generally have control over the organization, either explicitly or as a result of handling the daily operations. You might find a ceiling past which you cannot move.
An all-volunteer group suggests that there are more opportunities for leadership at the highest part of the organization. But donated time means that things might not move as smoothly as with the presence of a paid staff. The organization depends on people arranging their schedules and finding openings to keep things on track. If you're trying to learn to lead, removing one potential area of uncertainty could make the process easier.