The following guide will let you know how to join the Co-op in Serenitea in Genshin Impact. Pot easily.
When you go to your friend list and send a request, it will show that your friend is already in a co-op game and cannot apply.
Now all you have to do is press the friend detail, or if you are using any controller, you can press the R3 button or click on it.
After that, you have to click on Request to visit Serenitea Pot. After that, it will show you that your request is pending approval.
Now you have to wait for your friend to accept the invitation and then you can join it.