who were involved in preparing it?
A House and Senate conference committee.
Fifteenth Amendment to the United States Constitution
A House and Senate conference committee proposed the amendment's final text, which banned voter restriction only on the basis of "race, color, or previous condition of servitude." 
what does the fifteenth amendment to the constitution say It prohibits the federal government and each state from denying a citizen the right to vote based on that citizen's "race, color, or previous condition of servitude." when was it introduced? It was ratified on February 3, 1870.
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Fifteenth Amendment to the United States Constitution [SEP] Proposal
Both Southern and Northern Republicans also wanted to continue to deny the vote temporarily to Southerners disenfranchised for support of the Confederacy, and they were concerned that a sweeping endorsement of suffrage would enfranchise this group. A House and Senate conference committee proposed the amendment's final text, which banned voter restriction only on the basis of "race, color, or previous condition of servitude." To attract the broadest possible base of support, the amendment made no mention of poll taxes or other measures to block voting, and did not guarantee the right of blacks to hold office. This compromise proposal was approved by the House on February 25, 1869, and the Senate the following day.