You get a team lvl when you are for a longer period of time with the same members.
How about if you team up with prof X you get a specific team boost, resist etc. Combinations can be made by the already present Artil, Support, Infant, Control en Extrem subclasses.
If you team atleast 3 of those types you get bonus x add a forth you get bonus X and abit extra and if you have all 5 types in team you get the max team buff aura?