  We present X-ray and optical spectroscopic observations of twelve galaxy
groups and clusters identified within the XMM Large-Scale Structure (LSS)
survey. Groups and clusters are selected as extended X-ray sources from a 3.5
deg2 XMM image mosaic above a flux limit 8e-15 ergs/s/cm2 in the [0.5-2] keV
energy band. Deep BVRI images and multi-object spectroscopy confirm each source
as a galaxy concentration located within the redshift interval 0.29<z<0.56. We
combine line-of-sight velocity dispersions with the X-ray properties of each
structure computed from a two-dimensional surface brightness model and a single
temperature fit to the XMM spectral data. The resulting distribution of X-ray
luminosity, temperature and velocity dispersion indicate that the XMM-LSS
survey is detecting low-mass clusters and galaxy groups to redshifts z < 0.6.
Confirmed systems display little or no evidence for X-ray luminosity evolution
at a given X-ray temperature compared to lower redshift X-ray group and cluster
samples. A more complete understanding of these trends will be possible with
the compilation of a statistically complete sample of galaxy groups and
clusters anticipated within the continuing XMM-LSS survey.
