If you don't want to take bus, you will not go via Eupen. Unless you are taking Thalys or ICE, you need to change in Liège-Guillemins anyway.
If you are ok with Aachen-Liege-Brussel route, you need to buy tickets at Aachen Hbf to cover the part from Aachen to Liege. I don't know the price but you can consider the Euroregio ticket, on weekends it is 15 euro for two persons for traveling within one day.
For the part between Liege and Brussels, you have three options. Group ticket is not an option because that requires 15 people.
2. Rail pass: for over 26: 7.4 euro per single trip, must buy 10 in at one go, valid for a year.
can be used by multiple persons at the same time, can re-sell. http://www.b-rail.be/nat/E/tarifs/passes/rail/
