From what I've been noticing with the more obscure/hard to find credits, it seems that most of the people at Mario Club have been there for much longer than we think. For example, take Daisuke Nishikawa. Notice how his first official credit was in Breath of the Wild, right? Wrong. It was actually Nintendo Puzzle Collection, released way back in 2003.
Another example would be Aimi Ayashiro and Mahoto Sawada. Both first worked on games released in 2015, but looking at the in-game credits of Tomodachi Collection reveals that they've been working there since 2009, possibly even longer than that.
This is all pretty confusing, as most of the employees at Mario Club are hard to research due to them not having a presence on the internet. For whatever reason, they seem to be very low-profile. In some cases where website profiles were found, they don't list their join dates.
From what I've been estimating however, we can safely say that most of the people might have been working since Mario Club's creation in 1998. Note how there's also a large amount of people who started appearing from 2000 onwards.
Employees joining/leaving is impossible to track though, because as stated before, they never say when they joined or left on the rare instance that they have a public profile. There have also been cases where some people will disappear for several years from game credits, only to randomly reappear as if nothing happened at all. This could be attributed to most games not listing individual debuggers.