Actually in many cases you can use them interchangeably (if you mean a group of people who do something together, especially in sport), although 'drużyna' is a bit outmoded and its meaning is less universal. I would say 'drużyna' is very close to fellowship (and "The fellowship of the ring" was translated as "Drużyna pierścienia :-) ) while 'zespół' doesn't have such associations and usually means people working together (even just working on the same problem, like 'zespół specjalistów' who don't have to cooperate closely). Now, you must be careful with specific meanings of those words:: - if you talk about a music band it is 'zespół' - drużyna always mean people, while we can talk about 'zespół' of things, like 'zespół norm' , 'zespół ulepszeń' - in scouting we have 'drużyna' - and in the Middle Ages drużyna was a unit of warriors coming under a feudal lord (sergeants-at-arms)

Drużyna is beeing used only when it comes to sport or a group of people. Dobra z nas drużyna - we are good company (of friends for example). Zespół is more official. Oni tworzą zgrany zespół - They create a good team. Word zespół means also a band, musician band. In medical terminology this word is beeing used in "complex" meaning.