What does it mean if one of the group requests is "pending"?

It could be the case that one of the applications in the group is still processing. You can check the individual applications by searching for the details individually. To do that you would need the passport numbers, dates of birth, full names and nationalities of all the people in your group. If you have all that information at hand, then you can check the status of the people in your group individually.
ESTA applications can take up to 72 hours to process, and there may be a few details to double check before any confirmations are sent out. If one of your group members is pending - just check back on the application after the 72 hours have passed. Any pending applications will not delay those applicants in the group that have already received a response, and those who have had their application approved can use their ESTA to travel.
You can apply as a group of up to a maximum of 50 people, and you do not need to all travel together. The person assigned as the group contact will receive the information of the application and they need to be 18 years old or over. The group contact can also add or remove applicants from the group.