☛ State Library and Archive of Florida: “Christine McGuire in projection booth at Chris McGuire’s Theatre : Fort Lauderdale, Florida” photograph by Roy Erickson in 1970. Local call number: PR21194. Persistent URL. Same photo at the Flickr account of the State Library and Archives of Florida. No known copyright restriction.

Christine McGuire is standing by the preparation table of a projection booth, holding a 2000-foot reel of 35mm film. No, it’s not the nitrate making her smile like this. She’s most likely promoting her new theater franchise.
Christine McGuire (born July 30, 1926) is the oldest of the McGuire Sisters singing trio which was very popular in the United-States in the 50’s and the 60’s. From the National Broadcaster Hall of Fame website:

The sisters got their “big break” on the Arthur Godfrey Talent Scouts show in 1952 and remained there for seven years. They became one of the most well known vocal groups in the country in the 50s and 60s, appearing on top TV shows, performing at the White House for five presidents, and performing before the Queen of England. They were so popular that they, as representatives for Coca-Cola, received the highest fee in advertising history up to that time.

The McGuire Sisters stopped performing in public in 1968. The same year, Christine McGuire (presumably with the help of investors) launch a chain of theater franchises, “The McGuire’s Theaters”, much like Jerry Lewis would do the next year (he launch the “Jerry Lewis Cinemas” in association with the Network Cinema Corporation: see “Fantasy & Failure With Jerry Lewis Cinemas” by Christopher Crouch, March 28, 2009, as well as this original ad inviting investors to join the franchise).
It means that at one point there must have been more than one “McGuire’s Theater”. The one in which she was photographed (presumably a promo shot intended for investors) operated from 1968 to 1978 (I gather this info reading comments at Cinema Treasures). Below is another photograph taken the same day, at the same theater by the same photographer. Christine McGuire is easily recognizable in her white dress (also from the State Library and Archives of Florida, local call number no. PR21193):

More about the McGuire Sisters:

• AllMusic.com has a in-depth biography of the McGuire Sisters written by Bruce Eder.
• One of the McGuire’s most popular song is “Sugartime”. One can listen to it on YouTube: “Sugartime – The McGuire Sisters and Perry Como”
• On March 17, 1958 the McGuire Sisters appeared on the fromt cover of the LIFE magazine. The magazine also featured a four page article about them: get a PDF copy of the article or read it on Google Books.