server.R

library(shiny)
library(ggplot2)

shinyServer(function(input, output, session) {
  
  if (!is.null(session$user)) {
    data <- iris
    p <- qplot(Sepal.Width, Sepal.Length, data = iris, color = Species)
    output$ui <- renderUI({
      navbarPage(
        title = 'Permissions',
        tabPanel('Data',     dataTableOutput('ex1')),
        tabPanel('Graph',        plotOutput('ex2')),
        tabPanel('Authorized users only',  h3('Only users with permission will see this tab')))
    })
  } else {
    data <- iris[ , -5]
    p <- qplot(Sepal.Width, Sepal.Length, data = iris)
    output$ui <- renderUI({
      navbarPage(
        title = 'Permissions',
        tabPanel('Data',     
          h4("Login to see a species column"),
          dataTableOutput('ex1')),
        tabPanel('Graph',
          h4("Login to see clusters"),
          plotOutput('ex2')),
        tabPanel('Login',  a(href="./__login__", "Login here"))
      )
    })
  }  
  
  output$ex1 <- renderDataTable(data, options = list(iDisplayLength = 10))
  
  output$ex2 <- renderPlot({ 
    print(p)
  })
})

ui.R

library(shiny)

shinyUI(
  uiOutput('ui')
)