I always catch a syllable sounds like mai between 喺 and 一齊. I could get the exact syllable wrong but it should sound similar. The whole thing means 在一起, and 喺 means 在, 一齊 is 一起. So 1) what's the meaning or function of mai? 2) Is it necessary or optional? 3) Does it have other usages?
Using the information from the answers, I found an entry in 《香港粵語大詞典》that clears my first and third questions. From there I get its basic verbal meanings, particle functions that are grammaticalized from the verbal meanings, as well as how the meanings and functions developed.