Without a sample of the data, we're both flying blind. If it were me, I would have them send a sample Excel file so I can see the structure and begin the design(s) required.
As for the "tabs" (sheets) you can specify the sheet desired in the merge. If the data is in ranges in the same sheet, you can sort by the company name field, then by the last name of the participant. Then select one company's section and export that selection to a csv (and I always use tab delimited, UTF-8, .txt type of export). Or copy paste each company into a new sheet, naming the sheet for the company.
In lieu of obtaining a sample ahead of time, make one. And use more than one sheet to understand how it works.
Also, I hardly ever use ID's in-built merging. It an create a mess. I use Em Software/s Xdata (inside QXP) but InData works the same. Again, without seeing the data or understanding the design requirements, and not knowing what the client wants back, take the following with a grain of salt. I have done a similar job and I set up Master pages for the design variables. Then in XData, I apply the appropriate master page as the, in your case, company name changes. And one can do all sorts of other fun stuff during merging that goes far beyond this. I don't know if I would try learning InData at the last moment, though.
apparently, if you export as CSV in the app Numbers it goes to ****. You will need to boot your old pc to access excel and export there.
You can also likely copy the spreadsheet and paste into a text editor and obtain a tab-delimited file. That's how it works using Windows anyway.
Then save as a .text file and you should be good to go. Do make sure of the encoding when saving, though.