how did multicellularity arise from (animal like) protists?

Answer 1

It's generally believed that single-celled organelles such as hydrogenosomes and mitosomes (similar to mitochondria) were engulfed by other single celled organisms, via pinocytosis or someother engulfing technique. This then continued in a similar fashion to give rise to multicellular organisms

Answer 2

There's a lot of information in wikipedia on the subject.
http://en.wikipedia.org/wiki/Protists