The reason for your error with the SharePoint Admin Service is when you did this:
"i even did try to add users in the central admin ->manage service account etc. but that also in vain." I don't know WHERE you got the idea that this has anything to do with adding users. The Service Account for Central Admin is a SINGLE account that has Administration permissions to run SharePoint. You set it once when you install SharePoint, and you DON'T touch it again without a very good reason. I would be surprised if SharePoint works correctly at all, after doing that. You need to set the Service Account back to whatever it was before you changed it.
Now, your original problem, you can only add users who are in your Active Directory (AD) for your domain. (There are other sources other than AD, but I'm pretty sure you aren't set up that way.) You need to check Active Directory and see if the user exists. If they do, you should be able to add them. If not, they need to be added to AD first.