All of them can be used, depends on context:
a)To work as a team...
Para trabalhar como (uma) equipe...(geralmente virá com algo complementando,
veja ->) as in: To work as a team they merged they needed a plan.
To work as a team you need to develop your emotional intelligence.
b)Working as a team... (geralmente com algo antes, complementando. Não iniciando frase).
"Their interviews touched on six topics: elements they considered essential for teams to work; elements they considered potentially leading to failure of the team; benefits of working as team members; drawbacks of working as team members; benefits of team teaching for students; and miscellaneous thoughts."
"It is good to see people working as team and is good to see teams releasing commercial modules..."
" "The students gained valuable experience in working as team. "Working on a team, you've got to be open to lots of ideas," said Matt Stinemetze, " ''
c) Work as a team (also, it´s often comes with a complement, unless it comes as an order "Work as a team!"
Example of "work as a team" at http://ewh.ieee.org/r4/se_michigan/ems/mem.html
" Work as a team with the Event Coordinator, Greeters, Facility Liaison, and Speaker Host. Work any events you choose. We need several volunteers for each seminar."
Thus, it isn´t a case of right versus wrong.
You need to practice to get used to the structures. Hope it helps.