The absence of basic and obvious features like this still gives MindManager an amateurish feel. I'd expect this from freeware/shareware, but I expect more from the 10th version of a product targeted at enterprises.
You can also manually reorder markers in a group by creating a new marker group (of the same type) and dragging the existing markers to it in the required order. You can then rename the group and delete the original, or in the case of a mandatory marker group, delete the original markers and move them back in the right order from the temporary group. A bit long-winded, but it does work.
The order of groups is always based on the marker type they contain, so user-defined groups are inserted into the groups list in predetermined positions. You can delete any unused non-mandatory groups to tidy up the marker set.