This is clearly a case of someone writing their own version of a slang for a plural of the word "you".
In rural towns in northern USA and Canada, some people will actually say "yous" as in "How yous doing?" when addressing a group. This is incorrect usage and indicates a lack of education.
In your example of the Irish band, "yis" is most likely the same usage but spelled with an "i" to reflect their own accent or dialect (being Irish).
"Great to see you" is the correct usage whether you are addressing a single person or many people. "You" is both singular and plural.
Student or Learner