Formatting the elements of a panel

Question

This is a snippet of my interface (if choices = c () it means that the choice is managed on the server side):

sidebarPanel(
    selectizeInput(
        "s1",
        "Select s1",
        choices = c(),
        multiple = FALSE
    ),
    radioButtons(
        "r1",
        "Select r1",
        choices = c("A", "B", "C")
    ),
    radioButtons(
        "r2",
        "Select r2",
        choices = c("P", "R")
    ),
    selectizeInput(
        "s3",
        "Select s3",
        choices = c(),
        multiple = FALSE
    ),
    selectizeInput(
        "s4",
        "Select s4",
        choices = c(),
        multiple = TRUE
    ),
    selectizeInput(
        "s5",
        "Select s5",
        choices = c(),
        multiple = FALSE
    ),
    selectizeInput(
        "s6",
        "Select s6",
        choices = c(),
        multiple = TRUE
    ),
    selectizeInput(
        "s7",
        "Select s7",
        choices = c(),
        multiple = FALSE
    ),
    selectizeInput(
        "s8",
        "Select s8",
        choices = c(),
        multiple = TRUE
    ),
    actionButton(
        "b1",
        "Enter"
    )
)

I have two questions:

• Since some of the inputs are linked together, is there a way to group a couple of them into a subpanel with a subtitle? For example s7 and s8 are about the same choice and I would like this to be highlighted.
• Now the button is the bottom left of the panel. How can I move it to the bottom right?