What does "Va'ta'an lahem Miri'am" mean?
Rashi: It means that Miri'am sang and the women sang after her.
Why does the Torah write "Va'tya'an lahem Miri'am" and not 'Va'tashir' (as it does by Moshe and the men [See Targum Yonasan])?
Others explain that that is why they took drums (See Ma'ayanah shel Torah), and one can add that that is why the women left the camp to sing the Shirah in the desert.
See also Oznayim la'Torah, who, based on the fact that the Pasuk speaks in the masculine ("Va'ta'an lahem" and "Shiru", explains that she was merely encouraging the men to sing and not singing with the women.
What exactly did Miriam sing with the women?