Social categories and social aggregates are two different kinds of groupings in sociology. They have some things in common and other things that are different.
The main thing that they have in common is that members of both of these groupings do not necessarily know each other. You can be in the same social category and/or the same social aggregate as another person without having ever met that person.
Other than that, these are different ideas. People in a social category are tied together by some characteristic. For example, all American men make up a social category because they share their nationality and their sex. All high school students would also be a social category. By contrast, people in a social aggregate do not necessarily share any characteristics. The only thing that they share is that they are in a given place at a given time. All of the people in an airport at a given time make up a social aggregate even though they may not share any characteristics.
Thus, a social aggregate is a group of people who are in a given place at a given time while a social category is a group of people who share some characteristic. The members of these groupings do not need to know one another in any way.