Add custom group_id to customers on upload

I’m trying to add 5000 customers using the csv uploader. I used the API to add all 5000 customer codes, but when I try to use the csv uploader, it says:

 Value for group_id attribute contains incorrect value, see acceptable values on settings specified for Admin 

I’ve reindexed and cleared the cache, so why doesn’t it recognize the names as valid? Do I have to download all of the data, and write a script to replace the company names with their id’s?

Thanks