You do this in the _isAvailable method you implemented in your command Javascript, optionally you can also do this in the _isEnabled method.
In there you will have to check what Group the User belongs to. To get the current User and its Groups you can use the following:
var settings = Tridion.UI.UserSettings.getJsonUserSettings(true);
if (settings)
{
var groups = settings.User.Data.GroupMemberships;
}
Now the groups parameter will be be an object array containing the Groups the current User belongs to, they are formatted as JSON with a type, title and href (the TCMURI).