Fresh installation from git cvshead. Clicking on the "Groupings" tab causes the following error:
Fatal error: Call to a member function make_active() on a non-object in C:\xampp\htdocs\moodle2-hq\group\groupings.php on line 53

The "Overview" tab causes this one - same issue, different place:
Fatal error: Call to a member function make_active() on a non-object in C:\xampp\htdocs\moodle2-hq\group\overview.php on line 103

The corresponding lines in both files are identical:
$PAGE->settingsnav->get('courseadmin')>get('groups')>make_active();

With the menu structure as it currently stands, it can be resolved by changing both to:
$PAGE->settingsnav->get('courseadmin')>get(2)>get('groups')->make_active();

However, it seems strange that the "Users" node doesn't have a string key (it instead has a numeric key of 2, hence the ->get(2)). It may be preferable to switch it to a string ID and then use that in the chain instead. If "users" is already taken by another node, perhaps something along the lines of "courseusers" would suit?