Create Dynamic Group of Physical servers by Model ie (Dell PowerEdge)?

Try This,

Create a group with a dynamic query. this means any new nodes that match the query will automatically be added to the group, saves time and people forgetting to add them to the group.  I find that it takes about 10 mins for any new system to be added to the group.

Set the SolarWinds platform object to "Overall Hardware Status (Node)", 

Add 2x Conditions.

1. Manufacturer IS Dell Inc.
2. Model begins with PowerEdge

This should pull back some information for you. My test lab results.

Alternatively, you can create a custom Table/Query that displays the same info but with the NodeID and Serial Number of the box.
Here the SQL Query

SELECT aisi.[NodeID]
      ,aisi.[HostName] AS [Node]
      ,aisi.[Manufacturer]
      ,aisi.[Model]
	  ,aisi.[HardwareSerialNumber] AS [Serial Number]
  FROM [dbo].[AssetInventory_ServerInformation] aisi
  WHERE aisi.[Manufacturer] like 'Dell%'
    AND aisi.[Model] like 'PowerEdge%'
ORDER BY aisi.[HostName] ASC

To find what distinct manufacturers and model you have in the Asset Inventory Table.

SELECT DISTINCT
       aisi.[Manufacturer]
      ,aisi.[Model]
  FROM [dbo].[AssetInventory_ServerInformation] aisi

It does not look like this will work for assigning application monitors because its returning a object other than Nodes/Servers.

It's monitoring Hardware / Asset Inventory. You can create an application monitor based on a group.

If you could give more detail on what you are trying to monitor in your application and what you are hoping for, we might be able to help.
The initial request was how to create a group with Dell PowerEdge node in.

If you want to filter on Just Servers,  in the Group Query add a condition.  IsServer = True

The the SQL Query after the From.

LEFT JOIN [dbo].[NodesData] n ON n.NodeID = aisi.NodeID

the in the Where statement add 
AND n.IsServer = 1

SELECT aisi.[NodeID]
      ,aisi.[HostName] AS [Node]
      ,aisi.[Manufacturer]
      ,aisi.[Model]
	  ,aisi.[HardwareSerialNumber] AS [Serial Number]
FROM [dbo].[AssetInventory_ServerInformation] aisi
LEFT JOIN [dbo].[NodesData] n ON n.NodeID = aisi.NodeID
 WHERE aisi.[Manufacturer] like 'Dell%'
    AND aisi.[Model] like 'PowerEdge%'
	AND n.IsServer = 1
ORDER BY aisi.[HostName] ASC

It's monitoring Hardware / Asset Inventory. You can create an application monitor based on a group.

If you could give more detail on what you are trying to monitor in your application and what you are hoping for, we might be able to help.
The initial request was how to create a group with Dell PowerEdge node in.

If you want to filter on Just Servers,  in the Group Query add a condition.  IsServer = True

The the SQL Query after the From.

LEFT JOIN [dbo].[NodesData] n ON n.NodeID = aisi.NodeID

the in the Where statement add 
AND n.IsServer = 1

SELECT aisi.[NodeID]
      ,aisi.[HostName] AS [Node]
      ,aisi.[Manufacturer]
      ,aisi.[Model]
	  ,aisi.[HardwareSerialNumber] AS [Serial Number]
FROM [dbo].[AssetInventory_ServerInformation] aisi
LEFT JOIN [dbo].[NodesData] n ON n.NodeID = aisi.NodeID
 WHERE aisi.[Manufacturer] like 'Dell%'
    AND aisi.[Model] like 'PowerEdge%'
	AND n.IsServer = 1
ORDER BY aisi.[HostName] ASC