Your first doesn't work. It would be better as: When you decided to join this office/company, you must have known how we work here.
Your second would work in certain contexts.
Student or Learner
Can we say "when you would have joined this office you must have learned that how we work here.
There is 1 more question related to would have done "water would have been on the road a while ago"
In the first sentence, I am assuming as if I am talking to a senior person(Who is manager or something like that in the comapny that I willing to join), and he has asked me that if I do not know how to work or what is the work in the company then why do I wanna join it or how would I work here, so I just wanna tell him that when he joined it he also learned how to work there, so can we say "when you would have joined this office you must have learned that how we work here" for this scenario.
No, you can't use "would" there. Also, your sentence is a complete mess and is barely understandable. Please rewrite it, bearing in mind that "wanna" is not an English word.
Ok so what can be used instead of "Would have done"?