Hi Sharat,
You can achieve this by configuring the ACL settings for the user group.
In the
ACL>userGroup>Compose setting of the user group, configure these two settings:
'Allowed To Write New Post' > Yes
'Allowed To Publish Post' > No
http://take.ms/4CbtZ
The user group should be able to make a new post but would require approval beforehand by admin. Hope this helps.