That's pretty straightforward:
if (!(Get-NaUser eric -ErrorAction SilentlyContinue)) 
{
    New-NaUser eric -group users -password toolkit1!
} 
else 
{ 
    Write-Warning "User eric already exists"
}
or if you are confident your command won't fail for other reasons (e.g. bad group/password):
New-NaUser eric -group users -password toolkit1! -ErrorAction SilentlyContinue