"Expenditure groups" refer to specific categories or types of expenses. It is a way of grouping or categorizing different kinds of spending. Full definition
Independent expenditure groups spend money to influence the outcome of a city election, without the involvement of any candidate but may refer to candidates running for city offices. (politico.com)
Also note the possibility of a big Dem independent expenditure group forming. (epolitics.com)
As such, millions of dollars are being spent in the race from both the campaigns, party campaign committees and independent expenditure groups. (nystateofpolitics.com)