I've recently installed Grid Control 12c on RedHat 5.7 x86-64. After deploying agents via the mass deployment option EM provides, I added CRS clusters by doing a manual Add target from Grid Control itself. I was able to successfully create a Cluster target by grouping nodes for a Cluster, but most of the Cluster targets have no associated hosts and are in "Status Pending" after the CRS target configuration is completed and discovered.
Did you get any warnings when saving cluster target? Please verify that db plugin is deployed successfully on the hosts. Can you run the following query on EM repository and send results (please replace <cluster target name>).
select s.entity_name, s.manage_status, a.assoc_type, d.entity_name, d.manage_status
from gc_manageable_entities s, gc_manageable_entities d, gc_assoc_instances a
where s.entity_guid = a.source_me_guid
and s.entity_type = 'cluster'
and s.entity_name = '<cluster target name>'
and d.entity_guid = a.dest_me_guid;