There is no modal dispersion for single mode fiber but there are other dispersion like waveguide dispersion ( the input signal has different frequecies and each frequency sees different refractive index and hence different velocity, there by causing dispersion called as waveguide dispersion).
To conclude group velocity also makes sense for single mode fiber if have different frequecies at the input.
Group velocity is always pictured as a kind of average velocity (not average velocity in iteral sense, assume there 10 students in a class each working on the same project, each student will try to do his part but at different speed, so when you speak as a group you could average the rate of completion of project. Similarly, each mode tries to carry energy from end to other end contributing its part, so to get a picture of how fast energy is propagated you use the term group velocity)
This is the physical meaning of group velocity.
Regards,
Prakash.