ichidan verb
Honorific verb used when a person of higher status calls people together. Often used in historical or formal contexts.
王は家来たちを召し集めた。
The king summoned his retainers.
Neutral verb for gathering things or people. 召し集める adds an honorific nuance, implying the subject is a superior calling subordinates.
Compound of honorific prefix 召し (from 召す, 'to summon') and 集める ('to gather').