a highly active faction within a party or movement that presses for stronger action on a particular issue - informal
An old horse dealer's trick (recorded from the late 18th century) to make a broken-down animal look lively was to insert ginger into its anus. From this developed the metaphorical phrase ginger up meaning make someone or something more lively.
In the early 20th century the term ginger group arose to refer to a highly active faction in a party or movement that presses for stronger action about something.
1970 - New Society - The appearance of ginger groups to fight specific proposals is not necessarily a bad thing - particularly if the established bodies aren't prepared to fight.