Although the exact usage of the system is classified, NASA has a system called TDRS which is used by other government agencies for relaying data to ground stations.
To maintain communications with a ground station, a satellite would be stabilized in some manner and use a directional antenna. If the satellite is in a low earth orbit, the antenna essentially just has gain in the direction towards the earth. A tracking station acquires the satellite and communicates with it while it is in view.
How do two satellites in low earth orbit establish communications. Do they use directional antennas? For a satellite in typical low earth orbit, what is the radio horizon? How do they avoid interference amongst one another?