Checking out Google, I would say it is a Tiple. One definition says "A small instrument of the guitar family, similar to a ukulele, with 12 strings in 4 sets. Associated with the Andean region of Colombia"

Tuning seems to be at varisnce. There's this:


Tiple Strings

The Spanish word for 'treble' or 'soprano,' often applied to specific instruments. A small guitar of Spain, Columbia, Guatemala, Puerto Rico, and Venezuela. In Spain the tiple has fewer strings than the guitarra, and is strummed. The Colombian tiple is slightly smaller than the guitar, with four courses of metal strings tuned to the same pitches as the four upper strings of the guitar, but with the middle string of the three lowest courses tuned an octave lower. Courtesy of New Grove DMI © 1995.

and this:

The tiple (pronounced "Tee-Play") is a South American instrument, and comes in many shapes and sizes there, but C. F. Martin made only one size. This model is the bottom-of-the-line T-15, "just plain mahogany", with a rosewood fretboard. It is sometimes described as a "10-string Ukulele," but it's a lot more than that. It has four sets of strings, tuned left to right   F (high,low), Bb (high, low, high), D (high, low, high), G (high, high). That's a baritone tuning, but it has a thicker body than a Baritone, and a strong robust sound, like two or three ukes of various sizes playing together.   You can play it with finger or pick, depending on how much volume you want.

Bummer - the one I bought is not a Martin.

Anyway, thanks Deckman and others. All I've got to do now is work out how to incorporate it into the group.

Allan