I haven't tried it, but it looks like it should work on MyBB 1.4. Can't say anything about 1.5/1.6 as they may make that variable private.
There's a number of alternatives:
- If you don't need it right after everything else, you can hook admin_formcontainer_output_row instead.
- You might want to consider adding a single checkbox to the Miscellaneous row instead of adding a completely new row (still hook admin_formcontainer_output_row).
- Use a convoluted solution that I did for getting the XThreads options to appear where I want (I suggest against this though).
Also note that you'll probably also want to add the checkbox for the add group page as well.
Hope that helps.
EDIT: it seems it is private. Oh well, this elaborate hack will get around it...
Sorry have to go right now, so it may be wrong - just need to double check on that...