A coalition is an alliance of people or factions, sometimes only temporarily, for a common cause. A coalition can be formed by a group of citizens, united behind a common goal, that is referred to as a grassroots organization as, for example, the Christian Coalition. Here is the mission of this coalition:
The Christian Coalition (CC) has two central goals: to control the agenda of the Republican party by working from the grassroots up; and to train and elect pro-family, Christian candidates to public office.
NARFE, the national association of federal retirees and employees is another example of a coalition as it has the specific goals of protecting federal annuities and federal insurance programs and the economic security of present and retired workers.
Of course, there have been coalitions formed that have been very clandestine, such as the KluKluxKlan. A coalition of secessionists would certainly wish to be hidden from the legislation as would those involved in a tax protest movement. These coalitions are clandestine because they risk potential legal problems with the government.