Answer: Invasion
Explanation: Invasion could be said to mean an obstruction, a behavior or act which introduces or results in threat to a person's convenience or ease. Invasion does not necessarily have to be a war started by invading restricted territories, it could stem from simple acts of creeping into a person's personal space thereby introducing disturbance.
In the scenario above, the larger group due to their number might have dominated the smaller group in terms of intensity and loudness of their voice, thereby making it uncomfortable for the smaller study group to concentrate, thereby promoting them to vacate the space originally occupied by them.