  We discuss the extension of gauge-invariant electric and magnetic screening
masses in the Quark-Gluon Plasma to the case of a finite baryon density,
defining them in terms of a matrix of Polyakov loop correlators. We present
lattice results for $N_f=2+1$ QCD with physical quark masses, obtained using
the imaginary chemical potential approach, which indicate that the screening
masses increase as a function of $\mu_B$. A separate analysis is carried out
for the theoretically interesting case $\mu_B/T=3 i \pi$, where charge
conjugation is not explicitly broken and the usual definition of the screening
masses can be used for temperatures below the Roberge-Weiss transition.
Finally, we investigate the dependence of the static quark free energy on the
baryon chemical potential, showing that it is a decreasing function of $\mu_B$
which displays a peculiar behavior as the pseudocritical transition temperature
at $\mu_B=0$ is approached.
