From the book “The Key to Kanji” by Noriko Kurosawa Williams.
[The top 土 was a variant of a person, and 羊 “sheep” an animal that gives birth easily. Together with the bottom 辶 “to go forward” the term indicated “thing go smoothly”. Because people do things together, it also was used to mean “people”. The kanji 達 means “to reach, to attain”. The kanji is also used as a plural suffix for people in the kun-reading.]
Hope this helps. I personally prefer learning from the actual historical context of Kanjis, rather than made-up stories, but this might just be me.