Cisco Aironet Bridges support several roles : root, non-root, root-with-clients and non-root-with-clients.
When you use the -with-client role, the bridge accepts client associations. A WorkgroupBridge is seen as one (up to 8) clients. If you select the -with-client role, the bridge will accept association from the WGBridge.
The WorkGroupBridge supports only a maximum of 8 MAC@ (clients).