Performance of a group solving a cognitive task: How does it scale?
It's a big topic. The relationship between group size and performance on a cognitive task is going to vary by several factors.

Here are a few thoughts:


The form of interdependence adopted by the group on the task will matter. When everyone can just work independently (e.g., taking calls in a call centre), then it makes sense that output would increase almost linearly with increasing group size, in the absence of motivation or learning issues.
In the context of project teams, software teams, and so forth, I imagine the issues would be a lot more complex and that in some situations increasing group size might even reduce performance of the team. There a few articles cited here that discuss this.
A classic study in the context, albeit from a motor task, of the relationship between group-size and group performance is that by Ringelmann (see the wikipedia article) that looked at rope pulling power as a function of group size. The basic finding was that average pulling power declined with group size. This has been interpreted as evidence of social loafing, but their are also coordination issues as well.
Some tasks are heavily influenced by the performance of the weakest or strongest member. For example, a musical group will often sound bad if only one member plays poorly. Or other tasks it will be strongest member, as you say if you have are playing at a trivia night, and you have one person with all the right answers on the table.