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As -f takes an argument eg [-f file] you need a colon after it in the getopt argument:

while getopts mf:t: option

Then use $OPTARG to refer to the file (similar to what you did for -n):
f)...

[edit] I just realized your columns weren't made by WHO. I need to alter that echo too.

echo -e "User\tTTY\tDate\tTime"
who | cut -c1-7,9-15,23-25,28-34 | tr ' ' '\t'