Was it the right thing for me to tell my friends the truth about my lie to them and ask for their forgiveness, or is that considered exposing a sin that Allah has concealed? And was it necessary for me to confess for my repentance to be accepted?

Lying is forbidden by Islamic law. The liar must offer sincere repentance and increase acts of seeking forgiveness and righteous deeds. If the lie occurred in a normal conversation and did not infringe upon the rights of others, then seeking forgiveness from colleagues is not a condition for the completion of repentance, because the right belongs to Allah and not to them. Repentance can be made without informing them, especially if they do not know the person's identity.