Looking on the internet deeply has found these results:

in an NGO is the most popular phrase on the web.

at an NGO 1406,000 results on the web	in an NGO 1529,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I'm going to meet my classmates over there at an NGO. Before this, I worked at an NGO, and before that, I worked at Microsoft for a year, which felt like a decade. They must know I work for an NGO. She's working for an NGO there. Okay. I work for an NGO that represents undocumented aliens. Today I looked at a briefing from an NGO which said that it was weary of congratulating institutions. He works for an NGO in Mali.	Going to Brazil to work with an NGO in the Favelas. He works for an NGO in Mali. In Indonesia an NGO could not complete the distribution of 1500000 bottles of chemicals for treating water because of procurement difficulties. In some countries, an NGO can be formed with a membership of less than five people. DG ECHO provided funding to an NGO in Aceh to gather data concerning internally displaced people and integrate them into the Humanitarian Information Centre. If I have been informed correctly, Euroaid requires that the implementing organisation be an NGO in a developing country. For example, in preparing their boat building project in Aceh, an NGO worked closely with the community and succeeded in designing a project which met the needs of the beneficiaries. This man was in an NGO feeding center, being helped as much as he could be helped.

at an NGO

1406,000 results on the web

in an NGO

1529,000 results on the web

More popular!

I'm going to meet my classmates over there at an NGO.
Before this, I worked at an NGO, and before that, I worked at Microsoft for a year, which felt like a decade.
They must know I work for an NGO.
She's working for an NGO there.
Okay. I work for an NGO that represents undocumented aliens.
Today I looked at a briefing from an NGO which said that it was weary of congratulating institutions.
He works for an NGO in Mali.

Going to Brazil to work with an NGO in the Favelas.
He works for an NGO in Mali.
In Indonesia an NGO could not complete the distribution of 1500000 bottles of chemicals for treating water because of procurement difficulties.
In some countries, an NGO can be formed with a membership of less than five people.
DG ECHO provided funding to an NGO in Aceh to gather data concerning internally displaced people and integrate them into the Humanitarian Information Centre.
If I have been informed correctly, Euroaid requires that the implementing organisation be an NGO in a developing country.
For example, in preparing their boat building project in Aceh, an NGO worked closely with the community and succeeded in designing a project which met the needs of the beneficiaries.
This man was in an NGO feeding center, being helped as much as he could be helped.