It is common to choose a group of people to study the effect of certain things like the one given in this question. The selection of people is done after designing the sample size for a certain population. The design varies depending on whether the analysis is qualitative or quantitative.
The people who lie in the sample size actually contribute to the experiment are said to be part of it.
Therefore, if you are part of the study that is used to test the effectiveness of a new drug for treating the common cold. It means you are part of the experiment group.
You can learn more about the experiment groups and control groups from
