。
Let's get together again next year.
Literal
Next-year [also-も] again gather-let's.
集まる ('to gather') is intransitive — people coming together. 集まりましょう uses the polite volitional ~ましょう. もまた is a stacked emphatic sequence: も ('also') + また ('again'), suggesting 'next year too, again' — common for repeated annual events.