Below is the walk through for how to add another replica to the existing high availability group.
prerequisites
Back up the primary database and log , then restore to the new replica with NORECOVERY
From SSMs Availability replicas –> Add Replica

Connect to the existing replica




Above steps can be replaced with below script . However, the scripts needs to be executed under SQLCMD mode.

--- YOU MUST EXECUTE THE FOLLOWING SCRIPT IN SQLCMD MODE.
:Connect SGC8\SGC8SQL2014
USE [master]
GO
CREATE LOGIN [Stargate\sqlprodid] FROM WINDOWS
GO
:Connect SGC4,8001
use [master]
GO
GRANT CONNECT ON ENDPOINT::[Hadr_endpoint] TO [STARGATE\sqlprodid]
GO
:Connect SGC5\SGC5SQL2014
use [master]
GO
GRANT CONNECT ON ENDPOINT::[Hadr_endpoint] TO [STARGATE\SQLPRODID]
GO
:Connect SGC8\SGC8SQL2014
USE [master]
GO
CREATE ENDPOINT [Hadr_endpoint]
AS TCP (LISTENER_PORT = 5022)
FOR DATA_MIRRORING (ROLE = ALL, ENCRYPTION = REQUIRED ALGORITHM AES)
GO
IF (SELECT state FROM sys.endpoints WHERE name = N'Hadr_endpoint') <> 0
BEGIN
ALTER ENDPOINT [Hadr_endpoint] STATE = STARTED
END
GO
use [master]
GO
GRANT CONNECT ON ENDPOINT::[Hadr_endpoint] TO [Stargate\sqlprodid]
GO
:Connect SGC8\SGC8SQL2014
IF EXISTS(SELECT * FROM sys.server_event_sessions WHERE name='AlwaysOn_health')
BEGIN
ALTER EVENT SESSION [AlwaysOn_health] ON SERVER WITH (STARTUP_STATE=ON);
END
IF NOT EXISTS(SELECT * FROM sys.dm_xe_sessions WHERE name='AlwaysOn_health')
BEGIN
ALTER EVENT SESSION [AlwaysOn_health] ON SERVER STATE=START;
END
GO
:Connect SGC4,8001
USE [master]
GO
ALTER AVAILABILITY GROUP [pandaAlwaysOnDemo]
ADD REPLICA ON N'SGC8\SGC8SQL2014' WITH (ENDPOINT_URL = N'TCP://SGC8.stargate.com:5022', FAILOVER_MODE = MANUAL, AVAILABILITY_MODE = ASYNCHRONOUS_COMMIT, BACKUP_PRIORITY = 50, SECONDARY_ROLE(ALLOW_CONNECTIONS = NO));
GO
:Connect SGC8\SGC8SQL2014
ALTER AVAILABILITY GROUP [pandaAlwaysOnDemo] JOIN;
GO
:Connect SGC8\SGC8SQL2014
-- Wait for the replica to start communicating
begin try
declare @conn bit
declare @count int
declare @replica_id uniqueidentifier
declare @group_id uniqueidentifier
set @conn = 0
set @count = 30 -- wait for 5 minutes
if (serverproperty('IsHadrEnabled') = 1)
and (isnull((select member_state from master.sys.dm_hadr_cluster_members where upper(member_name COLLATE Latin1_General_CI_AS) = upper(cast(serverproperty('ComputerNamePhysicalNetBIOS') as nvarchar(256)) COLLATE Latin1_General_CI_AS)), 0) <> 0)
and (isnull((select state from master.sys.database_mirroring_endpoints), 1) = 0)
begin
select @group_id = ags.group_id from master.sys.availability_groups as ags where name = N'pandaAlwaysOnDemo'
select @replica_id = replicas.replica_id from master.sys.availability_replicas as replicas where upper(replicas.replica_server_name COLLATE Latin1_General_CI_AS) = upper(@@SERVERNAME COLLATE Latin1_General_CI_AS) and group_id = @group_id
while @conn <> 1 and @count > 0
begin
set @conn = isnull((select connected_state from master.sys.dm_hadr_availability_replica_states as states where states.replica_id = @replica_id), 1)
if @conn = 1
begin
-- exit loop when the replica is connected, or if the query cannot find the replica status
break
end
waitfor delay '00:00:10'
set @count = @count - 1
end
end
end try
begin catch
-- If the wait loop fails, do not stop execution of the alter database statement
end catch
ALTER DATABASE [AdventureWorks2014] SET HADR AVAILABILITY GROUP = [pandaAlwaysOnDemo];
GO
GO