By default you can't. You have to delete the share and reado it or you get the error "the name has already been shared".
Code:
C:\>net share Downloads2="C:\Users\Hali\Downloads" /grant:administrators,full /grant:users,read
Downloads2 was shared successfully.
C:\>net share Downloads2
Share name Downloads2
Path C:\Users\Hali\Downloads
Remark
Maximum users No limit
Users
Caching Manual caching of documents
Permission BUILTIN\Administrators, FULL
BUILTIN\Users, READ
The command completed successfully.
C:\>net share Downloads2="C:\Users\Hali\Downloads" /grant:everyone,full
The name has already been shared.
More help is available by typing NET HELPMSG 2118.
C:\WINDOWS\system32>net share Downloads2 /delete
Downloads2 was deleted successfully.
C:\>net share Downloads2="C:\Users\Hali\Downloads" /grant:administrators,full /grant:users,read /grant:everyone,full
Downloads2 was shared successfully.
C:\>net share Downloads2
Share name Downloads2
Path C:\Users\Hali\Downloads
Remark
Maximum users No limit
Users
Caching Manual caching of documents
Permission BUILTIN\Administrators, FULL
BUILTIN\Users, READ
Everyone, FULL
The command completed successfully.
You could download SetACL.exe and use that as described here.
For example to add then remove "Everyone":
Code:
C:\>SetACL.exe -on "Downloads2" -ot shr -actn ace -ace "n:everyone;p:change"
Processing ACL of: <Downloads2>
SetACL finished successfully.
C:\>net share downloads2
Share name Downloads2
Path C:\Users\Hali\Downloads
Remark
Maximum users No limit
Users
Caching Manual caching of documents
Permission BUILTIN\Administrators, FULL
BUILTIN\Users, READ
Everyone, CHANGE
The command completed successfully.
C:\>SetACL.exe -on "Downloads2" -ot shr -actn trustee -trst "n1:Everyone;ta:remtrst"
Processing ACL of: <Downloads2>
SetACL finished successfully.
C:\>net share downloads2
Share name Downloads2
Path C:\Users\Hali\Downloads
Remark
Maximum users No limit
Users
Caching Manual caching of documents
Permission BUILTIN\Administrators, FULL
BUILTIN\Users, READ
The command completed successfully.
The syntax of SetACL command is not particularly intuitive but is described well in their documentation here: SetACL Command Line-Version (SetACL.exe) - Syntax and Description if you want to play with share permissions (I don't )
This is another reason to set the share to everyone,full and control access through NTFS - you are just making an administrative headache for no benefit (that I can see) as access is granted on the most restrictive of NTFS and share permission anyway.