Here's a button (see here for what to do with the XML):
<?xml version="1.0"?>
<button backcol="none" display="both" label_pos="right" textcol="none" type="three_button">
<label>Share</label>
<icon1>#default:mapnetworkdrive</icon1>
<button backcol="none" display="both" label_pos="right" textcol="none">
<label>Share</label>
<tip>Share the current folder</tip>
<icon1>#default:mapnetworkdrive</icon1>
<function type="batch">
<instruction>@admin</instruction>
<instruction>net share {sourcepath$|nopath}={sourcepath$|noterm}</instruction>
<instruction>pause</instruction>
</function>
</button>
<button backcol="none" display="both" label_pos="right" textcol="none">
<label>UnShare</label>
<tip>Un-share the current folder</tip>
<icon1>#disconnectnetworkdrive</icon1>
<function type="batch">
<instruction>@admin</instruction>
<instruction>net share {sourcepath$|noterm} /DELETE</instruction>
<instruction>pause</instruction>
</function>
</button>
</button>
If you left-click the button then the current folder will be shared.
If you right-click it then the current folder will be un-shared.
It does this using the Windows "net share" command. It opens a DOS window so you can see if it was successful or not.
Note: Shares created using "net share" do not seem to update the folder icon overlay until Opus is restarted.