There was an issue with Groups v2 not working in grid mode with PostgreSQL (PGSQL) that has now been fixed and can be pulled from the latest development version of OpenSim (git version and not opensim v0.8.1.3a643e2 as is currently published on the download page).
There is a catch though with the fix that it destructively migrates a few database tables starting with the name os_groups_ so you have to backup the data in these tables before running the newer version OpenSim, and restore the data after.
We also found that in the table os_groups_membership the field AccessToken just contained blanks (spaces), but it must be given the value 00000000-0000-0000-0000-000000000000 in the data you import to successfully restore data to the updated table. This can be done by fixing the data in the database before backup, so the backup already has the correct data for this field, or it has to be edited into the SQL or XML you restore the table data from after the upgrade. I found the first option easiest to handle. Fixing data, backup and restore was done with Navicat.
There are still other oddities with the PGSQL driver in grid mode that is being worked on.
Since xmir is using PostgreSQL, groups are now working as intended on the grid.