Hi @Sanket Dange4,
It is not currently possible to have Groups/topics for 'admins' or moderators' use only. However, this is part of our next version already. Please click here14 to check out the timeline for reaching our next version.
You can restrict regular members from posting specific content types or all content types. To set this restriction go to Admin Settings> Content Types> Advanced Options (each content type has this option) and change "who can post?" to Moderators, it means that only admins and moderators are able to use this content type.
You do have the option to make your community read-only if you don't want your members to post at all on the site. Simply select Admin Settings> Settings> Access> Who can post? > No one it means that only admins are able to create content, not moderators or members.
I hope this helps!