As far as I know there is a going to be a class called arcanist which will be at launch. As with all classes they can also get a more specified role like the class conjurer can also switch to the job white mage for example. This job for arcanist is going to be summoner and will be implemented sometime after ARR launch.
I think I heard talk of four primals being possible to summon as your avatar (I seem to recall Ifrit and Titan at least). However it is only the essence or a part of the primal you summon if I understood it right because the real primals will be specific monsters you can fight out in the world. If you defeat them your free company will be able to summon them and use one time. This part is not tied to summoners from what I know though.
I am sure someone else will fill in what I missed or got wrong.
Edited, Nov 6th 2012 4:06pm by Belcrono