They are based in the bus transfer area for T5 to T3 and do the security clearance of those BA passengers transferring over to AA before departing from T5. It is reasonably new, less than a year old, but basically it allows those waiting for the bus to do the clearance rather than do it in T3 Flight Connections, where there is a dedicated hall for AA's transfer passengers. They only work during the relevant hours, so you won't find them down there later in the evening, but the AA desk remains in place there. The clearance is under DHS procedures for all Europe to USA flights on USA registered airlines - all AA passengers get it to a greater or lesser extent.