One obvious course of action when a project is behind schedule and you don’t want to cut scope is to expand the number of people on the project. The seldom works as expected.
Firstly, addition of new team members creates new work, the existing team members must spend time getting the new recruits up to speed, if the project has been running for a while, then this tacit knowledge transfer is not a trivial task.
Secondly, expanding the team size exponentially increases the number of channels of communication between people. In a team of 3 people there are three channels of communication A talks to B, B talks to C and C talks to A. By the channels of communication across a team have the relationship n(n-1)/2 where n is the number of team members. This means a team of 10 has 45, a team of 100 has 4,950.
Increasing team size may work, but it introduces its own complexity. Training new team members and the increase in channels of communication can make an increase in team size counter-productive.
This, and other concepts, primarily related to software development are explained in Brooks’ publication, The Mythical Man Month