I would like to hide a question group for selected users via a plugin. Can this be done in a plugin via php, with something like oGroup->hide(), without changing grelevance permanently? But could not find the proper function. Any pointer is appreciated.

Only in 4.X currently : www.limesurvey.org/manual/SetVariableExpressionEnd
I test with hidden attribute and relevance attribute in Question : it work great

With 3.X : i see only js solution (move next when group are shown) maybe someone have another idea.