computer outside of network & Terminal Service

i trying install terminal service friend @ store. server 2008 installed , have installed active directory , domain services 1 of roles. has 2 computers in store , 2 branches, each 1 computer. how add 2 outside computers active directory in order them used terminal service??? do have add them active directory , domain service in order connect terminal service??

here diagram of how network looks like

i working on server set friend has 3 different stores. don't have access centralize inventory, purchased quickbooks enterprise solutions. talked quickbooks in best way centralizes company file 2 branches (the 2 server not at) can connect main store server at. i've installed server 2008 enterprise. don't have domain, using workgroup. install terminal service server prompts me need in domain. create one. run more trouble.they want add 2 other computers outside network. not on domain nor on users , groups, ( 2 xp computers connected directly internet @ remote stores) how add them domain , users later add them ts remoteapp connection group???? please help!!!

here diagram show how network set up.

http://www.gliffy.com/go/publish/3736922/

Windows Server  >  Remote Desktop Services (Terminal Services)