The 'I' that witnesses being is beyond being all the more as its avatars in being are subject to infinitely rich vicissitudes of experience. You might say that it is its own ground, but to do so you need to re-evaluate the meaning of this term and everything related to it, including the notions of witnessing and beyondness. It's not that these words don't apply but that they undergo a radical pivoting. Being is framed in language but language is 'contingent' to the 'I'.  Starting here you can push further back into the source, what it is behind or apart from any and all 'I'-ness.