is it possible to pass multiple email address in one textbox and validate

Yes, but these emails must be separated by some delimiter.

a!localVariables(
  local!emails,
  local!invalidEmails: if(
    isnull(local!emails),
    {},
    joinarray(
      reject(
        isnull(_),
        a!forEach(
          items: split(local!emails, " "),
          expression: if(
            validateemailaddress(fv!item),
            null,
            fv!index
          )
        )
      ),
      ","
    )
  ),
  local!validation: if(
    isnull(local!invalidEmails),
    {},
    "Invalid email at " & local!invalidEmails & " position(s)"
  ),
  a!textField(
    label: "Enter emails",
    value: local!emails,
    saveInto: local!emails,
    validations: local!validation
  )
)

a!localVariables(
  local!emails,
  local!invalidEmails: if(
    isnull(local!emails),
    {},
    joinarray(
      reject(
        isnull(_),
        a!forEach(
          items: split(local!emails, " "),
          expression: if(
            validateemailaddress(fv!item),
            null,
            fv!index
          )
        )
      ),
      ","
    )
  ),
  local!validation: if(
    isnull(local!invalidEmails),
    {},
    "Invalid email at " & local!invalidEmails & " position(s)"
  ),
  a!textField(
    label: "Enter emails",
    value: local!emails,
    saveInto: local!emails,
    validations: local!validation
  )
)

Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function 'split' [line 7]: A null parameter has been passed as parameter 1.

Can you please help me with this

Sorry I forgot to add null check, I have updated the code above please copy again and check.

Thank you .It's working , is it possible to have line break 
it must validate like this

jason@test.com

amrutha@test.com

prasanth@test.com

Yes, just use paragraph field in place of text field and pass char(10) as delimiter in split function.

is it possible to implement the same with pickerFieldUsers?

What are you talking about? pickerFieldUsers component is used to select existing users.

Yes ,I want to enter the already existing users  as below format and then it should validate with already existing users with the Api / usergroups

jason@test.com

amrutha@test.com

prasanth@test.com 

No, this is not possible

User picker searches and selects user one by one

Hi   ,can you please explain me why you have used join and reject functions in this code