As historical examples you give forms of democracy and hierarchy, which are ways for a group to choose which action to take out of several options. But then you say that you don't want answers like IRV. I can't tell how to interpret your proposed distinction between "opinions" and "preferences".

Obviously, if we're talking about voting methods, then Arrow's Impossibility Theorem and some other related theorems may be of interest. Loosely speaking, they say that no deterministic way of structuring a group can simultaneously meet certain basic standards of fairness and consistency.

If all you want to do is solicit opinions but not vote on them, then I propose that the most straightforward way is best: ask for opinions. Let them be submitted anonymously to avoid going to Abilene.