Security and multiple sites in Appian

Assuming you simply need to hide/show components such as links on an interface, you can utilize a!isUserMemberOfGroup() to determine if the link should show based on group membership:

a!linkField(
  showWhen: a!isUserMemberOfGroup(
    username: loggedInUser(),
    group: cons!YOUR_ACCESS_GROUP_HERE,
    matchAllGroups: true    
  ),
  links: {/* link here */}
)

If that is not what you are looking for, please provide any additional details on your use case.

In addition to what Chris supplied already, which I agree with, I find it useful to base Site Viewership for each individual site on one specific "site viewership" group.  The group in question can contain other sub-groups as needed.  But it makes it relatively easy at all times to use a simple expression evaluating group membership for the current user, to determine whether they should have access to a particular site.

I have a series of steps to gracefully achieve it. 

1. Create a common application and the all users group of that app should have the all users group of all the other apps. 
2. Create a site in that app and use the code below to create the interface to navigate to different sites. 
   

   a!localVariables(
     local!columnsInARow: 4,
     local!configurations: {
       {
         icon: "cubes",
         title: "Manage Stock",
         description: "Manage your inventory in the system",
         showWhen: true,
         url: null
       },
       {
         icon: "dollar",
         title: "Manage Price",
         description: "Manage pricing of items you offer",
         showWhen: true,
         url: null
       },
       {
         icon: "users",
         title: "Manage Users",
         description: "Manage users within your organization",
         showWhen: true,
         url: null
       },
       {
         icon: "ad",
         title: "Manage Ads",
         description: "Manage advertisments across platforms",
         showWhen: true,
         url: null
       },
       {
         icon: "cogs",
         title: "More settings",
         description: "Access more settings of the system",
         showWhen: true,
         url: null
       }
     },
     local!visibleConfigurations: reject(
       a!isNullOrEmpty(_),
       a!forEach(
         items: local!configurations,
         expression: if(or(fv!item.showWhen), fv!item, null)
       )
     ),
     local!numberOfRows: ceiling(
       count(local!visibleConfigurations) / local!columnsInARow
     ),
     {
       a!forEach(
         items: enumerate(local!numberOfRows),
         expression: a!columnsLayout(
           columns: {
             a!forEach(
               items: index(
                 local!visibleConfigurations,
                 enumerate(local!columnsInARow) + 1 + (fv!item * local!columnsInARow),
                 null
               ),
               expression: a!columnLayout(
                 contents: a!cardLayout(
                   contents: {
                     a!richTextDisplayField(
                       labelPosition: "COLLAPSED",
                       value: {
                         a!richTextIcon(
                           icon: fv!item.icon,
                           color: "ACCENT",
                           size: "LARGE"
                         ),
                         char(10),
                         a!richTextItem(text: fv!item.title, size: "MEDIUM"),
                         char(10),
                         a!richTextItem(
                           text: fv!item.description,
                           color: "SECONDARY",
                           size: "SMALL"
                         )
                       },
                       align: "CENTER"
                     )
                   },
                   link: a!safeLink(uri: fv!item.url),
                   /* â`¢ Replace dynamic link with any other link type*/
                   height: "SHORT",
                   showWhen: not(a!isNullOrEmpty(fv!item))
                 ),
                 
               )
             )
           }
         )
       )
     }
   )

3. Now in front of showWhen you can use the function Chris proposed to check the security and the code will auto handle the visibility. 

There is just one overhead with this approach. You will need to update this interface everytime you add/remove a site in the system