This is what I learned so far from reading from the literature on behavior of social animals.
1. Sharing decisions with others can help, because several decision makers can pool their information and also eliminate individual error (Sumpter and Pratt 2009). As a result, the risk of making a bad decision decreases.
2. 'Wisdom of the crowd' effect or 'Many eyes' effect: Large groups are more likely to contain one particularly clever 'expert' fish, which guided the others, even though individuals within the groups might not differ much in their ability to make a correct decision (Ward et al. 2010).
3. However, decision speed might decrease with the number of decision makers, because sharing decisions requires communication between decision-makers. This communication process might also lead to information cascades, whereby decision makers no longer contribute independent information but instead amplify shared misconceptions (Conradt 2011).
4.. Typically, there is a trade-off between decision accuracy and speed, because more-accurate decisions usually require more information, which takes time (Franks et al. 2009).
5. Compared to decisions that are made by one dominant, democratic decisions are more beneficial because they tend to produce less extreme decisions, rather than because each individual has an influence on the decision per se (Conradt and Roper 2003).
6. Group decisions can be subject to manipulation by a self-interested and opinionated minority. Groups containing individuals who are uninformed about the decision being made are particularly vulnerable to such manipulation. But if the presence of the uninformed allows the majority to wrest control back by adopting the opinions of those around them, amplifying the majority opinion and preventing erosion by a manipulative minority, adding uninformed individuals to a group can facilitate fair representation and move towards a democratic decision (Couzin et al 2011).
Last and most interestingly, "two important factors that influence collective decision-making are information uncertainty and conflicting preferences...there remains one big gap in our knowledge: how do animals make collective decisions in situations when information uncertainty and conflict of interest operate simultaneously (Conradt 2011)?"