There's only one ground in LTspice, which is Node 0. If you want a second common point for part of the circuit you could use a named net.
Select Label Net from the toolbar or edit menu, or hit F4.
This will bring up a dialog where you can select COM, which will give you an alternate common symbol, or you can type in a name for a net and select which type (none, input, output, bi-directional) it is. Click OK and it will give you a symbol to place on the schematic. Use this symbol for all of the points you want to be your second ground.
Note that if you want to probe voltages relative to this new net you will need to set it as the the reference after running your simulation by right clicking it and selecting Mark Reference. This net will then be used as the reference (think black multimeter lead) for the voltage probe until you run the simulation again when you would need to set it again.