You can remove your own layer from a group recording by opening the performance, pressing on the three little dots, and then selecting "Hide". After a few minutes, your join will be hidden.
If you have any concerns contact our customer support team. Here's how: https://smule.zendesk.com/hc/en-us/requests/new
How do I remove another singer’s recording from my group recording?
You cannot remove another singer’s recording from your group song. If you want to maintain strict editorial control over your group recording, we recommend setting the recording to private and manually inviting the singers you want to join. When you are satisfied with the final outcome, close the recording and make it public for the world to enjoy.