Hi Ravi,
The "Add Group" option will allow you to select SCOM grops with objects and the other - "Add Object" can be used to select individual objects.
Please take into account that if yyou want all Alerts from a specific Computer, you have to select not only the respective computer object, but also the corresponding Health Service Watcher, like discussed here:
SCOM Alerts report
https://social.technet.microsoft.com/Forums/en-US/aa769b44-985b-44ef-ae38-ae50e02e4726/scom-alerts-report?forum=operationsmanagergeneral
Please try making some tests and generating reports, you cannot do anything wrong here, just try the different options to see how Reporting Works.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Stoyan Chalakov