There are jamaas who join a chama/merry-go-round..let's say 10 members and lets say each week they all contribute 2 k and give one member,ie 20k..how long will it take a member to get the 20k vs if they saved the 2k on their own weekly(tho requires discipline)?

10 weeks only and you have the 20k by saving alone!
With the chama what if one (or more) members who got their 20k mapema defaults after you have been faithfully contributing for weeks? Chamas and Saccos increase risk exponentially IMHO. All it takes is for a few bad apples to default and you are done in. The horror storos I have heard about this are legion. I would rather save for 2 years to reach my goal than take a risk with 10 people, any of whom can default at any time for any reason (or even die all of a sudden) and my money is gone! In which case a chama or sacco like that will be no different than the pyramid schemes that collapse that we have heard of for many years in Kenya.

I have a Constitution Template for a Chama/Investment Group. Contact me if you are interested.