Question: When you are talking to a 3rd person, fundamentally

> 
> (1) それは、**私達**の出来あげたものです。  
> 
>  (2) それは、**われわれ**の出来あげたものです。
> 
> 
> 

mean:

> 
> That is something that we all created together.
> 
> 
> 

With regard to using them as **first person, plural, possessive pronouns**, this a nuanced choice. `I'm not asking about using them as subjects, objects, topical nouns, etc. Just as possessive pronouns.`

* My feeling is that #1 is about taking more direct credit for what was accomplished? It sounds rude? Like you considered yourself the lead in a small group of 4 or 5 people?
* Even if you were the lead, always express to outsiders that *a team of equals*, among them yourself, accomplished something? That is choice #2?

Answer: 
* There is not such verb as 出来上げる, although 出来上がる (intransitive) and 作り上げる (transitive) exist.
* No one can possess a verb. That の after 我々/私たち is a *subject* marker, which can replace が only in relative clauses. See: [How does the の work in 「日本人の知らない日本語」?](https://japanese.stackexchange.com/q/12825/5010)
* 我々 and 私たち are always first person and plural pronouns, of course. And I don't know why you think they behave differently only when used as the possessive pronouns.
* Simply, 我々 is more formal and stiffer than 私たち. The number of people in the group doesn't matter. The level of confidence doesn't matter. Whether you were a lead or not doesn't matter, either. You can use 我々 when you do a presentation in a business setting, make a serious speech, take a job interview, etc. But recently more and more people tend to use 私たち in such a situation, too, because 私たち sounds milder. Politicians 100 years ago might have been using 我々 all the time, but we often hear politicians say 私たち today. Using 私たち won't be rude in almost any situation. Using 我々 in an inappropriate situation (eg. among close friends and referring to "we") is not rude, either, but it may sound a bit unfriendly, depending on the context.
Essentially, both mean "We". In my experience 我々 has the nuance of a "we" within a defined group: we (our class, our company, our country), where 私達 has no nuance. A very common use case is 我々日本人