Ronald Reagan was right to invade Grenada. At the time, there were political interests the United States had that made it the correct choice to invade. Although many people don't see the sense in doing this with a small country, socio-political alignments in the Caribbean made it necessery to do so.
The justification for invading Grenada was twofold. First, the moderate government had been overthrown by the People's Revolutionary Army of Grenada. Second, this revolution occurred while a massive airstrip was being built on the island nation. The United States had been practicing the Truman Doctrine (preventing the expansion of Soviet influence to new territories) since 1945. The invasion of Grenada to topple the pro-communist government and prevent the creation of a large refueling station for Soviet bombers off the coast of South America is fully in line with the Truman Doctrine. This was merely a continuation of the strategy that would eventually overstretch and bankrupt the Soviet Union and win the Cold War.
I do not think Ronald Reagan was right to invade Grenada. I do not think we had any place invading them, and still don't think it's justifiable. Also, the way we went about invading was wreckless, not well thought out, and seemed overly dangerous. He even admitted to his mistake there.
Ronald Reagan was wrong to invade Grenada. Why? Because the United States did not have a compelling reason for the war. Much like the lame excuse that Russia had to recently invade Crimea, the foreign nationals in Grenada were not at risk. Furthermore, the United States was not at a threat.