Well, according to definitions provided by dictionary.com, the difference seems to be that an agglomeration is “a jumbled cluster or mass of varied parts;” whereas, a conglomeration is “a cohering mass; cluster.” In other words, an agglomeration is “jumbled;” whereas, a conglomeration is more coherent.
So when referring to a group of people, an agglomeration could refer to group of people with varying goals, characteristics, etc., (i.e. kind of like a heterogeneous group of people). A conglomeration of people would; therefore, refer to a group of people, all with the same goal, characteristics, etc., (i.e. kind of like a homogeneous group of people).
For instance, a group of scientists sharing their research could be considered a conglomeration, (they’re all scientists, with a common goal of sharing research). However, a closer look may reveal that this group of scientists actually consist of scientists from various disciplines sharing the research from each of their respective disciplines; this would be an agglomeration, (biologists sharing biology research, physicists sharing physics research, etc.)
So whether or not a group of people could be considered to be an agglomeration or conglomeration also depends on how you choose to view the situation, (i.e. how much detail is considered).
The definitions of “agglomeration,” and “conglomeration,” also suggest that a conglomeration is more organized than an agglomeration. In other words, anything, (e.g. an event), that is organized to bring a group of people together is almost always referred to as a conglomeration. For example, a team meeting consists of a conglomeration of people.