Currently we only show when a group is used in:

• Permission Schemes
• Notification Schemes
• Issue Security Schemes
• Shared filters

A more complete list of locations where a group can be used is:

• Permission Schemes
• Notification Schemes
• Issue Security Schemes
• Shared filters
• Project Roles
• Global Permissions
• Custom field values
• Post functions in workflows
• Conditions in workflows
• Shared dashboards
• Filter subscriptions
• Plugins (we can't help check this one, because we don't know how plugins will use this)

We should extend this functionality to check as many places as possible because understanding the impact of changes to groups is very difficult for admins at this time.

You can work around this in part given the queries at: https://confluence.atlassian.com/display/JIRAKB/How+to+identify+group+usage+in+JIRA