The group that gained more political participation when amendments were added to the Constitution is:
African Americans.
The 15th Amendment, ratified in 1870, prohibited the federal and state governments from denying a citizen the right to vote based on "race, color, or previous condition of servitude," which aimed to increase political participation for African American men in particular following the Civil War. Additionally, amendments such as the 19th (granting women the right to vote) eventually contributed to broader political participation for various groups, but in the context of the question, African Americans specifically gained voting rights through constitutional amendments.