When working with DotNetNuke, we _highly_ recommend that each website be put into its own DotNetNuke installation.

However, if that is not an option, this is a workaround that will let you do that using the Plesk Control Panel.

You will need:
Plesk Control Panel
1 Shared IP Address
1 Dedicated IP Address for each "group"
1 "Master" domain name for each "group"
Multiple "child" domain names for each group.

Here is what you will need to do:
1)  For the Master Domain, make sure it is running on the Dedicated IP Address (Domains > [Domain] > Setup)

2)  Next, make sure that the master domain name is the default domain for the Dedicated IP Address (Server > IP Address > [IP Address])

3)  Now, you need to make sure that each child domain name is created in Plesk as a regular domain (not a domain alias).

4)  For each child domain name, select [Domain] > DNS Management in order to modify the DNS records.  You need to modify the "www.DOMAIN" and the "DOMAIN" records to point to the ip address of the msater domain name.