  We present initial results from the Polarisation Sky Survey of the Universe's
Magnetism (POSSUM), analysing 22,817 Faraday Rotation Measures (RMs) with
median uncertainties of 1.2 rad m^-2 across 1,520 square degrees to study
magnetised gas associated with 55 nearby galaxy groups (z less than 0.025) with
halo masses between 10^12.5 and 10^14.0 M_sun. We identify two distinct gas
phases: the Intragroup Medium (IGrM) within 0-2 splashback radii and the
Warm-Hot Intergalactic Medium (WHIM) extending from 2 to 7 splashback radii.
These phases enhance the standard deviation of residual (i.e., Galactic
foreground RM-subtracted) RMs by 6.9 +/- 1.8 rad m^-2 and 4.2 +/- 1.2 rad m^-2,
respectively. Estimated magnetic field strengths are several microGauss within
the IGrM and 0.1-1 microGauss in the WHIM. We estimate the plasma beta in both
phases and show that magnetic pressure might be more dynamically important than
in the ICM of more massive clusters or sparse cosmic web filaments. Our
findings indicate that "missing baryons" in the WHIM likely extend beyond the
gravitational radii of group-mass halos to Mpc scales, consistent with
large-scale, outflow-driven "magnetised bubbles" seen in cosmological
simulations. We demonstrate that RM grids are an effective method for detecting
magnetised thermal gas at galaxy group interfaces and within the cosmic web.
This approach complements X-ray and Sunyaev-Zel'dovich effect methods, and when
combined with Fast Radio Burst Dispersion Measures, data from the full POSSUM
survey, comprising approximately a million RMs, will allow direct magnetic
field measurements to further our understanding of baryon circulation in these
environments and the magnetised universe.
