Am I sinful for attending a marriage contract ceremony where it was later discovered that the bride was pregnant out of wedlock by the same man?

There is no sin upon whoever attends a marriage contract without knowledge of the spouses' situation. However, it must be noted that it is not permissible to marry a woman pregnant from adultery until after her repentance and after she has given birth, due to the Prophet's (peace be upon him) saying: "A pregnant woman should not be approached until she gives birth, nor a non-pregnant woman until she has had one menstrual cycle."