How to use the output from multiple "rounds" of selectizeInput() in R Shiny?

Question

In my app, users upload experimental data. I'm trying to use selectizeInput() to allow users to select columns from the table to create different experimental groups, i.e. column 1 and 2 might be group 1, and column 3 is group 2. This is the code I have so far:

server(input, output) {
  values <- reactiveValues(
    numExpCr = 1
  )

  observeEvent(input$cr_exp, {
    values$numExpCr <- values$numExpCr + 1
  })

  output$cr_exp_gr <- renderUI({
    crExp_list <- lapply(1:values$numExpCr, function(i) {
      expname <- paste("Exp", i, sep="")
      selectizeInput("sel_exp", label = "Select samples for new biological group", choices = colnames(exp_dff()), options = list(create=TRUE), multiple = TRUE)
    })

    do.call(tagList, crExp_list)
  })
  sel <- reactive({
    req(input$sel_exp)
    return(exp_dff() %>% select(input$sel_exp))
  })

  output$sel <- renderDataTable(sel())
}

The problem I'm running into is that the function works fine for creating a single group, but I want users to be able to create multiple. I'm unsure of how to go about that though.