Add space before uppercase letter

Using sed, and assuming you don't want a space in front of the word:

$ sed 's/\([^[:blank:]]\)\([[:upper:]]\)/\1 \2/g' file.in
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Tell Me Who You Are

The substitution will look for an upper-case letter immediately following a another non-whitespace character, and insert a space in-between the two.

For strings with more than one consecutive upper-case character, like WeAreATeam, this produces We Are ATeam. To sort this, run the substitution a second time:

$ sed -e 's/\([^[:blank:]]\)\([[:upper:]]\)/\1 \2/g' \
      -e 's/\([^[:blank:]]\)\([[:upper:]]\)/\1 \2/g' file.in

Perl, using lookbehind and lookahead zero-width regular expressions:

$ perl -pe 's/(?<=\w)(?=[A-Z])/ /g'  file.in 

Tell Me Who You Are                    ## TellMeWhoYouAre
I Am A Regular Expression User         ## IAmARegulaExpressionUser

This version is also separating consecutive uppercase letters.

sed -r -e "s/([^A-Z])([A-Z])/\1 \2/g"

Add space between a letter that is not an upper-case letter and a letter that is an upper-case letter