It looks like only individuals are put into the sysapproval_approver table and it does not seem to reference what group the are a member of directly.  It looks like you would need to search sysapproval_group and get the sys_id from there of what you are looking for approvals on so you can then search for the members of that group who have rejected the request in the sysapproval_approver table.  Note it only looks like the table sysapproval_group table gets populated if the workflow activity of Approval - Group is used to set the approvers.  The group in the sysapproval_approver table seems to be a reference to which group was set as the approver in the sysapproval_group table.  Hopefully this was not to confusing as I confused myself was I was looking at the two table and testing what populated each from the workflow.