Seems this field is similar to tags field where Seblod does some additional queries in background to get the info. You will need to add some joins and order part manually by using search join and search query fields.
yes but what is strange is the fact that in the SEARCH tab the user_groups field is correctly understood the and jointure is done. if you activate the debug you can clearly see that the Query makes a JOIN with #__user_usergroup_map
however in the ORDER tab the same field is not recognized.
Actually what we are trying to do is to SORT the users according to their groups and make a kind of GROUP BY
list of the users from group1
list of the users from group2
list of the users from group3
Maybe this cannot be achieved without a free query ?