At this stage you should have accurate performance metrics, IT support for the service levels and escalation procedures. Now you are ready to meet with employees to get their feedback. What employees should participate? As a suggestion, your most critical employees might be the best. They are great people to have in your customer task force because they really want to see IT meet their needs. They will not be afraid to give you their opinions. They will certainly question your current response time report. Their memories are long and they remember how hard it is to get the support they need to do their job. They will usually be fair. If you can get them to agree to the SLA, you can get anyone to agree. Also, it is better to get any arguments and concerns out of the way now before you go public with the SLA.
Here is a great White Paper on Implementing Service Level Agreements. See https://www.givainc.com/wp/it-help-desk-software-cloud-whitepapers.cfm