I've got one... Rolleinar 1 in Bayonet I mount - for my Rolleiflex T.
If you're looking to buy one on ebay or elsewhere, you already have the info you need - you're looking for Rolleinar I, which comes in two parts, as Helen said, but sometimes in THREE parts (I also have one of those).
The later ones come in two parts, and are usually marked as "Rolleinar 1" on the larger (top) part.
The earlier ones come in three parts, two thinner and one thicker, and are marked "Rolleiparkeil 1". One thinner part couples with thicker and goes on the top (viewing) lens. The other thinner part goes on the lower (taking) lens.
Any of those models will also have a dot on the upper part, which has to be at 12:00 position for proper parallax compensation.

The self-portrait in my gallery was done with the Rolleinar. Apart from an unusually ugly subject , I'm quite satisfied with the optic quality of those close-up lenses. Just be sure that the glass is not scratched or damaged in any way, as with any other optical device...

With Rolleinar, the DOF is greatly reduced, which is something I like in portraits.
Your subjects may not be thrilled with their portraits, though - as with any portrait taken with "normal" length lens. You know, the nose seems bigger, etc...