What is the interpretation of seeing death and the narrowness of the grave, then exiting through a window and seeing a place resembling Paradise, after performing Istikhara prayer regarding joining the Ahmadiyya community?

We have no knowledge regarding the interpretation of dreams. The Ahmadiyya (Rifa'iyya) Tariqa is a Sufi order that includes polytheistic innovations (bid'ah shirkiyyah), and it is not permissible to join it or to seek Allah’s guidance concerning it. This applies to every sect that contravenes the beliefs of Ahl al-Sunnah (definition) wa al-Jama'ah. What you heard in your dream about the narrowing of the grave for not joining the order is from the deception of Satan. Choose Islamic groups whose methodology is based on theah a'mthodology of Ahl al-Sunnah wa al-Jama'ah in creed and practice.