All IPCC group reports are finalised via political negotiation except for one group. WG1 is the scientific group, all the others refer back to the WG1 report for factual information, the other groups argue about how to present those facts in their own working group(WG). In 25yrs of incredibly intense scrutiny, nobody has ever found a factual error in the final versions of a WG1 report. That really is a very robust outcome and a credit to the scientists involved.
Only nations that donate to the IPCC budget get a vote on the other reports, last I checked there were ~135 nations who together represent pretty much every political view in the rainbow, it takes a long time for them to agree. The IPCC budget is $5-6M/yr, nobody who actually works on the reports is paid a dime by the IPCC, all of the scientists involved DONATE their time. Their financial accounts are on their web site. Try finding the accounts for an anti-science no-think-tank such Senator Inhofe's barking dog - the heartland institute.