Following powershell functions can be used to create Groups, Remove permissions from a group in a list and adding permissions to a group in a list
function Create-SPGroupInWeb

{

                param ($Url, $GroupName, $PermissionLevel, $Description)

                $web = Get-SPWeb -Identity $Url

                if ($web.SiteGroups[$GroupName] -ne $null)

                {

                                Write-Host "Group $GroupName already exists!" -foregroundcolor Red

                }

                else

                {

                                $web.SiteGroups.Add($GroupName, $web.Site.Owner, $web.Site.Owner, $Description)

                                $group = $web.SiteGroups[$GroupName]

                                $roleAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($group)

                                $roleDefinition = $web.Site.RootWeb.RoleDefinitions[$PermissionLevel]

                                $roleAssignment.RoleDefinitionBindings.Add($roleDefinition)

                                $web.RoleAssignments.Add($roleAssignment)

                                $web.Update()

                                Write-Host "Group $GroupName created successfully" -foregroundcolor Green

                }

                $web.Dispose()

}

function Remove-SPPermisssionFromListGroup

{

                param ($Url, $ListName, $GroupName, $PermissionLevel)

                $web = Get-SPWeb -Identity $Url

                $list = $web.Lists.TryGetList($ListName)

                if ($list -ne $null)

                {

                                if ($list.HasUniqueRoleAssignments -eq $False)

                                {

                                                $list.BreakRoleInheritance($True)

                                }

                                else

                                {

                                                if ($web.SiteGroups[$GroupName] -ne $null)

                                                {

                                                                $group = $web.SiteGroups[$GroupName]

                                                                $roleAssign = $list.RoleAssignments.GetAssignmentByPrincipal($group);

                                                                $roleDefinition = $web.RoleDefinitions[$PermissionLevel];

                                                                $roleAssign.RoleDefinitionBindings.Remove($roleDefinition);

                                                                $roleAssign.Update();

                                                                $list.Update();

                                                                Write-Host "Successfully removed $PermissionLevel permission from $GroupName group in $ListName list." -foregroundcolor Green

                                                }

                                                else

                                                {

                                                                Write-Host "Group $GroupName does not exist." -foregroundcolor Red

                                                }

                                }

                }

                else

                {

                                Write-Host "List $ListName does not exist!" -foregroundcolor Red

                }

                $web.Dispose()

}

function Add-SPPermissionToListGroup

{

                param ($Url, $ListName, $GroupName, $PermissionLevel)

                $web = Get-SPWeb -Identity $Url

                $list = $web.Lists.TryGetList($ListName)

                if ($list -ne $null)

                {

                                if ($list.HasUniqueRoleAssignments -eq $False)

                                {

                                                $list.BreakRoleInheritance($True)

                                }

                                else

                                {

                                                if ($web.SiteGroups[$GroupName] -ne $null)

                                                {

                                                                $group = $web.SiteGroups[$GroupName]

                                                                $roleAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($group)

                                                                $roleDefinition = $web.RoleDefinitions[$PermissionLevel];

                                                                $roleAssignment.RoleDefinitionBindings.Add($roleDefinition);

                                                                $list.RoleAssignments.Add($roleAssignment)

                                                                $list.Update();

                                                                Write-Host "Successfully added $PermissionLevel permission to $GroupName group in $ListName list. " -foregroundcolor Green

                                                }

                                                else

                                                {

                                                                Write-Host "Group $GroupName does not exist." -foregroundcolor Red

                                                }

                                }

                }

                $web.Dispose()

}

The functions can be called like this:

# Create web level groups

Create-SPGroupInWeb $Url "Finance Members" "Read" "Finance Members group"

# Remove contribute permissions from groups in Customers List.

Remove-SPPermisssionFromListGroup $Url "Customers" "Contracts Members" "Contribute"

Add-SPPermissionToListGroup $Url "Customers" "Contracts Members" "Read"