My first question would be, were you ever a Catholic before you tried to convert your friend to another religion? I was raised Prot. and can, and did, convert people as a Prot. Of course I didn’t convert them to the Catholic Church, why would I? I wasn’t Catholic myself. What I did, under those circumstances was not a sin because I was converting them to Jesus, the best I knew how.
If I tried to convert someone to a non-denominational church now, for example, now that I am a Catholic, then yes, it would be a mortal sin. But then again, why would I do that, now that I know the Truth.
If you confessed to the best of your ability, then, no, I wouldn’t think the confession is invalid. Just mention it the next time you go. Tell the priest what you meant by “led others into grave sin”, and the circumstances of the leading. Remember, God never holds you responsible for what you don’t know. (That’s not permission to never know anything so you aren’t held responsible for anything, of course!

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Also, if you were baptized when you became a Catholic, then everything you ever did prior to your baptism was washed away. So if the incident happened before baptism, then it never even needed to be confessed at all. It was washed away with the waters of baptism.
Kris