Release : DX NetOps Spectrum 10.X
There is no way to undo a model deletion in Spectrum. The only way to bring these models back without recreating them is to restore your database from a previously saved database that had the User Groups and Users contained within it.
Ideally, you should restore the backup on all SpectroSERVERs in a DSS. However, you could restore the MLS SSdb and cycle tomcat and all OC clients (to refresh the cache). Launch a new client, open the Users Tab and you will see the deleted User Group. Select it, and go to the Landscapes tab and add it to all of the other landscapes. This will repopulate the User Group and Users to the other landscapes.