In most cases of hyphenated compound nouns you add an "s" to the part of the word being pluralized. So "mothers-in-law" or "masters-at-arms."
The "s" can also be added at the end such as in the case of "six-packs" of soda.
In an open form compound noun, the standard pluralizing rule is to add an "s" to the main noun. For example, "half brothers" or "sergeants major" or "post offices."
Closed form compound nouns tend to change on an individual basis. "Passersby" remains passersby. Cupful can be "cupsful" or "cupfuls" (though cupfuls is the preferred spelling). Makeup stays makeup, softball becomes "softballs," but firefly changes to "fireflies" by pluralizing the second word in the compound noun. In the case of closed form nouns, the easiest solution is to consult a dictionary.
One of the trickiest aspects of a compound noun is making it plural and possessive. As a general rule, if one of the nouns in the compound noun is attributive, pluralizing it keeps the possessive trait. For example, a consumer group is a compound noun that refers to a group belonging to a consumer. So a "consumers group" is the plural form.