The Secular Student Alliance will resume their meetings on January 13th, 2013. Again at the Bernhard Center in the Martin Luther King room 214 at 4:00 PM. Erdman says big their primary wish is to bring more people into the group, even those who are not atheist or agnostic. The more diverse, the better.

how about this one? It's saying my last sentence is a fragment.. how can I improve that?

This is also a fragment:

"Again at the Bernhard Center in the Martin Luther King room 214 at 4:00 PM."

This doesn't make any sense. Did you reverse a couple of words?
"Erdman says big their primary wish is to bring more people into the group, "

How do you think you can correct the last sentence?

*Again the meeting will take place at the Bernhard Center in the Martin Luther King room, room 214 at 4:00 PM.

*Erdman says they hope more people will join the group

The more diverse the viewpoints are the better?

That's still a fragment.

Erdman wants diverse points of view.

what about the others I corrected?

Omit "Again" at the beginning of the sentence. The rest is correct.

To improve the last sentence and make it a complete sentence, you can consider adding a subject and a verb. One option could be: "Having a more diverse group will make it better." This way, the sentence includes a subject ('Having a more diverse group') and a verb ('will make'). This revised sentence is now a complete sentence.