Installing a Meter Socket and Safety Switch
This can be confusing when first dealing with panels.  

How main panels work

The utility supplies only 2 hots and a neutral.  That is coming from the meter.  

Your main service panel supplies 1-2 hots, neutral and safety ground to all circuits.  

Where does safety ground come from?  From a local pair of ground rods.  

There is also a Neutral-Ground equipotential bond inside the main service panel, which bonds neutral and ground.  In an ideal world, these are on two separate buses, connected by a thick wire you can put a clamp meter around.  However, it is perfectly acceptable to use one bus-bar for both inside the main panel only.  

This disconnect switch is functioning as your main panel

So it needs to be wired like a main panel.  

This will be where the ground rods attach, and this is where the neural-ground bond must be.  As such, you can simply put all neutrals and grounds on the same bar.   Everywhere else, they must be separate.