Incoming links: Caenorhabditis elegans
One of the simplest known seems to be: en.wikipedia.org/wiki/Trichoplax
www.u-tokyo.ac.jp/focus/en/articles/a_00220.html "The simplest multicellular organism unveiled" from 2013 mentions Tetrabaena socialis.
Then of course: Caenorhabditis elegans is a relatively simple and widely studied model organism.
Nicole King (UC Berkeley, HHMI) 1: The origin of animal multicellularity by iBiology (2015)
Source. - youtu.be/1v6cgSkiHik?t=513 multicellularity is polyphyletic, e.g. evolved separately on plants, fungi and animals.
- youtu.be/1v6cgSkiHik?t=668 describes how unicellular organism choanoflagellates form colony, and how animals are characterized by certain key types of cellular interaction: adhesion, communication, regulation (cell differentiation) and extra cellular matrix production