Should I join an NGO between the period of my final year graduation exams and the CAT exam?

It is  always better to have a full time job than being a part of an NGO for a certificate. 

CAT guarantees nothing. Sometimes even after all the hard work, something or the other might go wrong resulting in losing your opportunity and end up regretting not taking up a job offer.

Joining an NGO is definitely a good idea. But, it should be for a genuine cause and you should put in sincere efforts. There is no point in joining an NGO for the sake of it or just for a certificate. Because it won’t do you any good.

If you are planning to be a manager in the future, learn to balance CAT preparation and a full time job. If you commit to this and work hard diligently, you will be able to get calls from the IIMs as well.