Some of the online groups will also allow relatives to join if they provide the information about the person currently attending an SA. I don't really think there is much in the way of specific information that would only be applicable to grandparents.
I don't know of any groups specifically for grandparents. However, the USAFA parent's group in our state welcomes all family members to join and be a part of the group. We have many grandparents who attend meetings along with member parents and participate in the group. They are also welcome at the Wisconsin reception during Parent's Weekend and attend the football games with us.