The subject is sponge because that is what is the main point of the sentence is.
The correct answer is "Few in the band have learned all of their music yet". When a verb agrees with its pronoun subject, it needs to sound natural. "Neither" and "either" always agree with a verb in singular form, and "anybody" agrees with a verb in the singular form as well.
They usually have their roots in C, 'Latin and Greek'. Such words are - formula, amateur, arcane, incentive...