I need to add to form to a folder that I am sharing with someone else

I need to add to form to a folder that I am sharing with someone else. I can see his folder on my account. But I cannot add anything to it. Can you help me please.

Do you want to move your form to the folder that has been shared with you? If that is the case, simply drag and drop the form into the folder, a message will popup letting you know that you are actually transferring ownership of the form:

If you want to keep ownership of your form, then you can add the account username you want to share your form with, under your account as a "sub-user": https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-account-User 

Let us know if you need more help.