In 2367, when there arose a dispute among the miners, the planet requested a mediator from the Federation to settle the dispute. The mining shuttle Nenebek was carrying Captain Jean-Luc Picard and Ensign Crusher to the planet for mediation when the vessel suffered a thruster failure and crash-landed on Lambda Paz. After the shuttle disappeared, Pentarus V was searched. (TNG: "Final Mission")
This planet was only mentioned in dialogue.
"pen-TAR-us" was the pronunciation for Pentarus from the script pronunciation guide. 
The settlement's relation with the Federation is unclear: on one hand it is implied that decisions like might be reached in this conflict could influence Federation Law, but on the other hand Dirgo makes it clear that the settlers "don't have the Federation's resources".
Pentarus station might be associated with this planet, or even be the settlement, as Pentarus V appeared to be the only point of interest in the system.
According to Star Trek: Star Charts ("United Federation of Planets I"), in 2378, Pentarus V was listed as a Federation member. It was, as well, identified as a Federation planet by the Star Trek Encyclopedia (4th ed., vol. 2, p. 132).