New players are welcome to join the Frontier server, it's just that Frontier isn't updated to the latest version because we're waiting for our mods to all be updated. Because we run several server-side mods to make our admin jobs easier and provide some additional benefits to players that vanilla minecraft doesn't offer - we have to wait for all those mods to be updated before we can use them (and we have to test them). We have been delayed with this for a few reasons, one being some mods have been slower to update and need additional testing to see if they work, we have been extra busy with Xmas and getting the FTB server set up.
You're also welcome to join the FTB server (which is where most of us are playing at the moment, so Frontier might be very lonely for a while), that is modded, but very easy to install. My beginner's post should help give info about how to use it.
If you've updated your client to the latest version, and want to go on Frontier, you can always use MC nostalgia to backdate your Minecraft to 1.3.2. I personally use Minecraft Profile Manager to handle multiple versions of Minecraft, so I can play the newer versions for Single Player with any updated mods, and I can keep older versions for connecting to the TOG servers or using any mods that haven't been updated. (So what you would do is create a new profile in Profile manager, which will be the most recent version, then backdate that with Nostalgia to 1.3.2)
(Post about using Profile manager, and info on MC Nostalgia is at the bottom of that post)