In Texas (CenterPoint Energy) from the meter can to the disconnect, I have three wires, HOT, HOT and Neutral. In the disconnect I need to connect the Neutral to the ground however the Neutral bus bar only has an opening for one 4 ought wire. Do I need to purchase another ground bar and install it in the disconnect and Bond them together? I know they need to be bonded there, how do I physically do it?
In answer to your question. Yes you do need to add another ground bar to bond the neutral and the ground wire at the first means of disconnect and you will need to attach your ground you brought in under ground. Also as @Tyson has pointed out you will need a driven ground rod at the disconnect. What everyone else is saying is that you will need to isolate the neutral from the ground at any panel down stream. As far as what method you use to bond this disconnect you probably want to run that through the AHJ. I like @Paul Logan's advice but you need to use a four barrel lug. Neutral in, neutral out, grounding electrode conductor, and grounding wire out.
As far as I can see the disconnect is service rated. It's just a fused disconnect instead of a main or enclosed breaker.