setMembership(userName, role) this method
      sets the membership details for a user.
    If the user is not already a member of the site, then they are added with the role given. If the user is already a member of the site, then their role is updated to the new role.
Only a site manager can modify memberships. There must be at least one site manager at all times.
Parameters
- userName
 - A string representing the user's username.
 - role
 - A string representing the role for the user.
 
Example
    var site = siteService.getSite("swsdp");
    if(site){
        var authorityName = "joe.user";
        var role = "SiteContributor"; // "SiteManager", "SiteCollaborator", "SiteContributor", "SiteConsumer"
        site.setMembership(authorityName, role);
        ...
   
    }