a person who teams up with another person towards a common goal
There are several words that means a group of people with a common interest/purpose/goal/aim etc. These words might depend on the context as well:
union: a number of persons, states, etc., joined or associated together for some common purpose: student union; credit union.
coalition: an alliance or union between groups, factions, or parties, esp for some temporary and specific reason
league: An association of states, organizations, or individuals for common action; an alliance.
coterie: a small exclusive group of friends or people with common interests; clique
association: a group of people who have joined together because they have similar interests or aims
club: an organization for people who have a common interest in a particular activity or subject
For the second question:
effector - one who brings about a result or event; one who accomplishes a purpose
Hope you find it helpful. With regards :)