Is driving on the street at a speed exceeding the legal limit—which led to death—considered suicide, and what should be done to seek forgiveness for the deceased?

What your friends did by speeding is recklessness and rashness, which is not permissible according to Islamic law and violates traffic laws. The driver must abide by these laws, as they are among the things that the ruler restricts what is permissible for the public interest. Their action is not considered suicide unless they intended to kill themselves. You can pray for them, give charity on their behalf, perform good deeds and dedicate the reward to them, including Hajj and Umrah (definition) if you have already performed them for yourself.