Cyanobacteria are an example of prokaryotes that, while not truly multicellular, may at times exhibit similar behavior.
They can form colonies and, within groups that share walls, specialize into different functions such as photosynthesis or nitrogen fixing behavior. This enables the colony to adapt to changing conditions that, while still made up of individual organisms, collectively appear to function similarly to a single multicellular one.
With that said, there aren't any examples of truly multicellular organisms within either domain. Just as an ant colony is not a single organism, cyanobacteria colony specializations are not examples of organs.
They can form colonies and, within groups that share walls, specialize into different functions such as photosynthesis or nitrogen fixing behavior. This enables the colony to adapt to changing conditions that, while still made up of individual organisms, collectively appear to function similarly to a single multicellular one.
With that said, there aren't any examples of truly multicellular organisms within either domain. Just as an ant colony is not a single organism, cyanobacteria colony specializations are not examples of organs.