Collective decision-making is studied from various perspective, like the following:

Social Choice Theory, which is about aggregation of judgments or preferences, such as by various ways of voting.(1,2,3)
Public Choice Theory, which focuses on decision-makers as self-interested agents.(4)
Individual decision-making and its fallacies. (5)
Theory of representation (6) and delegation (see liquid democracy in the list of examples.)
The study of decision-making based on reason, knowledge, and wisdom, like in noöcracy and epistocracy. (7, 8)
Argumentation theory, that is, logic, rhetorics, public discourse, and so on.
Deliberation (as in citizen's assemblies) and group dynamics.
System change, as in organisations.
Global governance

These areas would have to be supplemented by relevant portions of politicology, sociology, psychology, anthropology, e-democracy, organisation theory (bureaucracy) and so on, not to mention problem analysis and goal setting.

Arrow, K.A. ; Sen, A.; Suzumura, K. (eds.) (2001) Handbook of Social Choice and Welfare, Elsevier/North-Holland https://www.ier.hit-u.ac.jp/Common/publication/DP/DP417.pdf
List, C. (2013) Social Choice Theory in: Stanford Encyclopedia of Philosophy https://plato.stanford.edu/entries/social-choice/
Amadae, S.M.: Impossibility Theorem. https://www.britannica.com/topic/impossibility-theorem (Especially the bottom line.)
Rowley, C. K.; Schneider, F. (2004) The Encyclopedia of Public Choice, Springer. https://link.springer.com/book/10.1007%2Fb108558
Gilboa, I. (2011) Making better decisions, Wiley-Blackwell.
Tangian, A. (2014) Mathematical theory of democracy, Springer.

7. Vandamme, P.-E. (2019) What’s wrong with an epistocratic council? Politics, Vol. 40, nr. 1.

8. Sakai, R. (2020) Mathematical Models and Robustness Analysis in Epistemic Democracy:

A Systematic Review of Diversity Trumps Ability Theorem Models. Philosophy of the Social Sciences, Vol. 50, nr. 3.

Decision theory: examples