Call of Duty: Modern Warfare made a big change this year by not including an always active mini-map/radar, and while the community has been debating away whether or not they should add it back, one thing is certain that many can agree on is that there definitely should be a way to know where teammates are.
Source: N4G PC Call of Duty: Modern Warfare Will Have Teammates Feature Outlines So you Can See Them Through Walls