How can i give user an access to Sandbox?

If that user is active in the sandbox here is what you need to do

• Change their email address as sandbox email addresses are masked (This is technically optional, but necesary if they need to reset their passowrd)
• Make sure they use the sandbox URL   http://test.salesforce.com
• Make sure they use the proper username as its different for production.  Usually, its there standard priduction username with ".sandboxname" appended.  So if my production username is chris@mydomain.com, and I have a sandbox called "devsand", then my username in the sandbox would be chris@mydomain.com.devsand