On 17 March 1842 Emma and 19 other women met with Joseph on the second story of the red brick store in Nauvoo to form the Nauvoo Female Relief Society. Their objective "according to Joseph Smith was 'that the sisters might provoke the brethren to good works in looking to the wants of the poor, searching after objects of charity and in administering to their wants; to assist by correcting the virtues of the female community.'"  The sisters in attendance would unanimously elect Emma as the president to "preside over the Society. . .just as the Presidency, preside over the church." 
"In just two short years, membership [grew] from the original twenty to approximately 1,341 members." Today there are over 4 million women involved in the Relief Society in more than 160 different countries and provinces.  The hymn, "Now Let Us Rejoice," was sung at the close of the first Relief Society meeting. 
The minutes of the Kirtland Temple dedication indicate that "Now Let Us Rejoice" was sung to the tune "Hosannah," the same tune used for "The Spirit of God." 
See transcription of Now Let Us Rejoice as printed in Emma's 1835 hymnal.