Is there sin upon one who was the cause of a young man and a young woman meeting, which led to them committing indecency several times, knowing that he did not intend harm and did not commit adultery himself, and he is regretful of that?

The one who repents from a sin is like one who has no sin. And Allah loves those who repent and forgives all sins. Regret and supplication for forgiveness may be a means by which Allah overlooks your transgression, but what you did was an act of disobedience and sin. Mere seclusion, looking, and touching between non-mahram individuals are sins. It would have been more fitting for you to fear Allah and forbid evil instead of inviting to it. Now, be firm in your repentance.