I know there's some sort of list regarding the tier a Pokemon is in. However, when comes to a team, which consists of 6 different tiers Pokemon, how to determine which tier this team is in?
The Pokemon that is ranked in the highest tier out of your team if the tier your team is.
So if you had a team of Pokemon from NU and a Dragonite (who is in OU), that team would be OU just because of Dragonite. This is to keep powerful Pokemon in the higher tiers with other Pokemon of similar strength, to keep a balance between the tiers.
So yeah, pretty simple really.