Group delay is defined as the rate of change of transmission phase angle with respect to frequency. The units work out to time when the angle is in radians and frequency is in radians/time (seconds, nanosecond, picosecond or whatever is convenient, depending on the length of the path). When group delay is extracted from S-parameters, unless the network is a perfect measurement of a perfect transmission line, there will be variations over frequency. But within a small amount of bandwidth, group delay is usually nearly constant. Thanks to Martin, for keeping us on out toes!