Can a Government servant be a member of NGO?

Yes,  Central Government employee can be a  simple member of an NGO provided that NGO in not  indulged in political or any activities which is detrimental to sovereignty of the India. As per Deptt. Of Pers.& Trg. O.M. No. 11013/5/88-Estt. (A) dated 11.07.1988 
1.Instances have been brought to the notice of this Department where Government servants seek prior permission for becoming members of socio-religious bodies, the objectives of which are claimed to be aimed at social reforms and religious awakening etc.
2. According to Rule 15 of the Central Civil Services (Conduct) Rules, 1964, no previous sanction of the Government is required for a Central Government servant to undertake honorary work of a social or charitable nature but he should discontinue taking part in such activities, if so directed by the Government. Prior permission is essential only if the Government servant seeks to hold an elective office. As regards participation in purely religious activities, the freedom to profess and practice any religion is guaranteed under the Constitution of India itself. Since, however, the Constitution of India is based on the principle of secular state, the Government servants, while they are free to profess and practice any religion in their private lives, should so conduct themselves in public as to leave no room for any impression to arise that they do not subscribe to the secular philosophy of the State.
3. Some of the organizations and movements claiming to aim at social and religious reforms, may have some attributes of sectarian or communal nature. It will, therefore, be desirable on the part of the Government servants to be very cautious in associating themselves with any organization or movement the activities of which are liable to be construed as sectarian or communal in nature. It is not possible to give an exhaustive list of such activities or of the organizations and movements whose aims and objectives may be objectionable. The responsibility for the consequences of the decision to join any organization and participating in its activities will rest with the employee himself. It is, therefore, the duty of the Government employee who wishes to join any organization or association to satisfy himself that its activities and objectives are not of such a nature as are likely to attract action under any of the provisions of the Conduct Rules. In the circumstances, any plea of ignorance or mis-conception as to the Government’s attitude regarding participation in the activities of such organizations would not be tenable.
[Deptt. Of Pers. & Trg. O.M. No. 11013/5/88-Estt. (A) dated 11.07.1988]