© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. We use the first 100 deg2 of overlap between the Kilo-Degree Survey and the Galaxy AndMass Assembly survey to determine the average galaxy halomass of~10 000 spectroscopically confirmed satellite galaxies in massive (M > 1013 h-1 M⊙) galaxy groups. Separating the sample as a function of projected distance to the group centre, we jointly model the satellites and their host groups with Navarro-Frenk-White density profiles, fully accounting for the data covariance. The probed satellite galaxies in these groups have total masses log 〈Msub/(h-1 M⊙〉≈ 11.7-12.2 consistent across group-centric distance within the errorbars. Given their typical stellar masses, log 〈M*, sat/(h-2 M⊙)〉 ~ 10.5, such total masses imply stellar mass fractions of 〈M*,sat〉/〈Msub〉 ≈ 0.04 h-1. The average subhalo hosting these satellite galaxies has a mass Msub ~ 0.015 Mhost independent of host halo mass, in broad agreement with the expectations of structure formation in a Λ cold dark matter universe.