It's reasonably common for events like social dances to require vaccination. This usually looks like requiring attendees be "fully vaccinated", "up to date", or "fully vaccinated and boosted", and cashes out to the people at the door checking for a primary series plus potentially a booster.

A bit over a week ago, however, the CDC put out updated guidance, including:

Monovalent (original) mRNA COVID-19 vaccines will no longer be recommended for use in the United States.

And:

CDC recommends that everyone ages 6 years and older receive an updated (bivalent) mRNA COVID-19 vaccine, regardless of whether they previously completed their (monovalent) primary series.

This reads to me that if someone is not yet vaccinated and decides to become so, they'll be given a single bivalent mRNA dose.

Events that require vaccination need to decide how to handle this, and it would be good to think about before someone shows up at the dance! At the dance I help organize we phrase it as requiring a booster if eligible, but if it's your first shot then it's not boosting anything? And some dances are even more explicit about requiring something you can't get anymore:

All students, instructors, and staff who attend in-person classes and/or events must be fully vaccinated against COVID-19. Fully vaccinated means that it has been at least two weeks since your second dose of Pfizer or Moderna vaccine or first dose of Johnson & Johnson vaccine. Starting January 2022, individuals must have received a COVID-19 booster shot if they received Johnson & Johnson only or if their 2nd dose of Pfizer or Moderna was more than 6 months ago. —Blues Union

(Not trying to say anything here on whether or when I think events should require vaccination.)