NGO

No, unless he has obtained prior permission of the concerned departmental head, the employee cannot be an executive member of the NGO

YES. 

It is your constitutional citizen right. 

Exception is only for security forces.

There is no bar.  Anybody can join a NGO and participate in non-profit service activities in any capacity. 

Since the details required in this query are incomplete so our replies are also limited. 

You did not tell that you are working in which department.

Exception for secutity forces in veiw to keep them impartial for religious activities such as they cannot be a member in Durga Pooja  Committees, Ganpati Utsav Committee etc.

If the query contents all details of all related aspects than only a complete information can be replied.