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Using the Feature Assistant Extension on Chrome or Firefox, you get a Quick Entry option to “Start Merit Badges”. I don’t recall whether or not it allows you to assign the counselor at the same time.
However, the extension also generates a “Connect to Scouts using CSV” option on the dashboard of merit badge counselors who are listed in Scoutbook. The MBC should be able to connect to all of the scouts at once using that.
ETA: The Feature Assistant Extension is described here:
