You can quickly update multiple teachers' access to student groups by going to Settings > Students and Staff > Staff, then clicking the grey Mass Update Staff Groups & Teaching Assignments button.
Use the drop-down menus for each staff member to select the student groups they need access to. You can use the Check All and Uncheck All options and/or manually select whole grade levels or individual groups. If your school uses Academics, you can also assign staff members to the courses they need to enter standards mastery data.
These changes are saved automatically.