How to get the currently logged in users into an application

There is a function for this

docs.appian.com/.../fnc_people_loggedinuser.html

Agree with Stefan there isn't really anything OOTB to satisfy your requirement here.  However you may be able to utilize the Execute Stored Procedure plugin to record users interacting with the form, then display the stored data within the form itself, to show which users are logged in.

Note, the plugin's function fn!executestoredprocedure() is not designed to modify data, which is noted on the App Market page.  However, I have used it to successfully to record auditing/logging type data in similar situations.  I cannot speak to if the function will be modified in the future to physically prevent this.  But, this example will update a login data set each time the a!textField() is interacted with.  In a short time playing with it today, I was not able to get the auto-refresh to work (intended to have it update every 30 seconds, automatically), but it is refreshing correctly based on refreshOnVarChange, in which you can set any variables that users interact with.

Created on Appian 20.3, MSSQL database.

Table:

CREATE TABLE [dbo].[test_login_audit](
	[id] [int] IDENTITY(1,1) NOT NULL,
	[user] [varchar](100) NULL,
	[loginTime] dateTime NULL
) ON [PRIMARY]
GO

Stored Procedure, results in one row per user with the latest form interaction time:

CREATE PROCEDURE [dbo].[test_usp_login_audit] 
	@user varchar(100),
	@loginTime dateTime
AS
BEGIN
	DECLARE @count int = (SELECT COUNT(*) FROM dbo.test_login_audit WHERE [user]=@user)
	
	IF @count = 0
	BEGIN
		INSERT INTO dbo.test_login_audit ([user],loginTime) VALUES (@user,@loginTime)
	END 
	ELSE
	BEGIN
		UPDATE dbo.test_login_audit set loginTime = @loginTime WHERE [user]=@user
	END


	SELECT loginTime from dbo.test_login_audit WHERE [user]=@user
END
GO

Interface, this will log an interaction on load and any time the Test Field is modified:

a!localVariables(
  local!testField,
  local!audit: a!refreshVariable(
    value: fn!executestoredprocedure(
    dataSourceName: "java:comp/env/jdbc/YOUR_DB_CONNECTION",
    procedureName: "dbo.test_usp_login_audit",
    inputs: {
      {name: "user", value: tostring(fn!loggedInUser())},
      {name: "loginTime", value: now()}
    }
  ),
  /*refreshInterval: 0.5,*/
  /*refreshAlways: true,*/
  refreshOnVarChange: local!testField
  ),
  {
    a!textField(
      label: "Audit Result",
      value: local!audit,
      readOnly: true
    ),
    a!textField(
      label: "Test Field",
      value: local!testField,
      saveInto: local!testField
    )
  }
)