Prokaryotic multi-cellular life exists, but is very primitive.
The earliest multicellular life forms were all prokaryotes, first appearing around 3 to 3.5 billion years ago (Grosberg and Strathmann, 2007) and exhibiting cellular diversification starting over 2 billion years ago.
They aren't terribly complex, of course, looking something like this:
Ultimately, prokaryotes are good at forming into globs, and can even manage globs in specific shapes, but their evolutionary repertoire doesn't extend much beyond that, probably due to an inferior information storage system when compared to the DNA nuclei of eukaryotes. It's likely that this pattern would be seen in extraterrestrial life, as well, with complex multicellular life arising from creatures with the capacity to store lots of information in a DNA-like structure. These creatures wouldn't be eukaryotes, per se, since they don't share a common ancestor, but would be the alien equivalent.