We can grant permission to blogger to add new user to their blog in wordpress MU. To grant the permission we can need make some settings from wordpress MU site admin section. So please check with these steps:
1) Login into wordpress MU site admin.
2) Click on options from site admin section.
3) Select radio button [yes/no] for Add New Users – To Allow blog administrators to add new users to their blog via the Users->Add New page.
4) Click on update options to apply changes.