After working with Symantec support my issue has been resolved and it the issue was with the DB.
We turned on verbose logging in tomcat and tried to add a computer to a client group - within the tomcat logs we were able to see undefined object errors returned from the SQL query.
Unfortunatly the fix was was to remove SEPM and reinstall it. Luckily I was able to export most of my settings through the export domain command but I did lose my licenses, LDAP settings, and some of the administrators for some reason (added back maybe 1/3 of the admins I had defined).
All of my clients have been checking back in and everything looks good in the client group views now.
Thanks for working with me on this - I hope your resolution is a bit easier.