In the "Groups" tab ("Groups module") you create groups and add users to them.
You can then use the groups other places in Group-Office. For example you can share a file directory with a group - or single users.
>but i was struggle to how can we assign which module for particular "user group".
You do this in the "Modules module"
>And how can I delete Group "Everyone"..
You can't. Think of it as "Everyone" is not a group - it is *everyone* - so it should not be deleted.