I am Central Government employee is it required for me to take a permisson from my office to give voluntary service as a RTI Volunteer or to be a member of any NGO.
The question asked by you should be answered in the back ground of CCS(CCA) rules. A copy of the same is available here.
The CCS(CCA) rules on the issue of joining an association says
No Government servant shall join or continue to be a member of, an association the objects or activities of which are prejudicial to the interests of the sovereignty and integrity of India, or public order or morality.
There are a list of such bodies which are banned.