I'm running HEAD updated. I've created one course and, from the page, I've selected the "groups" icon (in the admin block). It invokes:

/moodle_head/group/index.php?id=2

And is dropping this error:

Warning: require_once(/Users/stronk7/cvs_moodle/moodle_head/lib/json/JSON.php) [function.require-once]: failed to open stream: No such file or directory in /Users/stronk7/cvs_moodle/moodle_head/group/index.php on line 14

Fatal error: require_once() [function.require]: Failed opening required '/Users/stronk7/cvs_moodle/moodle_head/lib/json/JSON.php' (include_path='.:/usr/local/php5/lib/php') in /Users/stronk7/cvs_moodle/moodle_head/group/index.php on line 14

that prevents groups from working completely.