Let’s try to sort out the noun/adjective endings with a simple
(silly) sentence: first singular,
‘A friend of a friend provides (to) a friend a fine thing’ [= food,water, etc.]
Remember, genitives, like adjectives can modify any noun in the sentence (or serve as object of a preposition = out of/from); I just put it under the nominative (subject) to make a convenient graphic.
Now convert that to the plural: ‘Friends (masc. & fem.) of friends provide to friends fine things (Remember ‘things’ =neuter plural)
Don't forget 'equational' sentences (A=B);
in those, you have nominative on both sides of the equation:
'girls are strong': κόραι (εἰσιν) ἰσχυραί. Etc.
So, 'Friends.... are a fine party'