In the Roman Empire, there were several factors to become a citizen.

• People who were from the Latin states were granted citizenship.
• Slaves who were freed became citizens.
• Foreigners who lived in conquered lands were given partial citizenship.
• Children born to a legionary were denied citizenship.

Rights given:

• The right to vote (In the Republic, women were citizens but couldn't vote)
• The right to make contracts
• The right to have a legal marriage
• Citizens couldn't be punished by death unless they were accused of treason.

After AD 212, all freemen in the Empire wrre granted citizenship from the Constitutio Antoniniana.

References

• Roman citizenship