As the title asks: the Compass/BeiDou-2 system constellation includes five satellites in GEO, while most of their satellites are in MEO at about half the altitude. Do these provide a ranging signal, or if not what is their function?
TL;DR: It depends. The actual answer depends which generation of the BeiDou system one is referring to.
The excellent Springer Handbook "Global Navigation Satellite Systems", edited by Teunissen and Montenbruck, has a section describing the BeiDou system. In fact, there are several generations of the system: BDS-1, BDS-2 and BDS-3:
BDS-1: There were two satellites in GEO, launched in the year 2000 and positioned at 80'E (BeiDou West) and 140'E (BeiDou East). User navigation relied on a two-way signal exchange between the receiver device and the GEO satellites. The GEO satellites relayed the signals to the Master Control Station (MCS) to offload the processing for determination of the user's location and clock correction.
The localisation of a user terminal in BDS-1 is based on turn-around signal travel time measurements initiated by the MCS. [...] the MCS first emits an interrogation signal to the two satellites, which is subsequently broadcast to the users in the service area via the outbound transponders of the two satellites. The user receives the interrogation signal and sends its response signal with the user's service request back to the satellite.
[...] After the BDS-1 was established, the first generation of BDSBAS (BDSBAS-1) was embedded in 2003. It broadcasted augmentations for the GPS navigation signal using the GEO satellites.
BDS-2 (current system): There are five satellites in GEO (geostationary orbits), launched between the years 2009 and 2012, and positioned at 58.75'E, 80'E, 110.5'E, 140'E and 160'E. There are a further five satellites in IGSO (inclined geosynchronous orbits, centred on 118'E and 95'E), and six in MEO. Figure 10.14b clearly shows that the signal structure for the BDS-2 GEO satellites includes a PRN code at 2.046MHz and Navigation data at 500Hz. In terms of the GEO satellites:
Apart from the standard positioning and timing service available in the entire service area, the BDS-2 open service signals also support an improved, satellite-based augmentation system (SBAS)-like positioning service based on near-real-time corrections transmitted through the GEO satellites.
[...] In BDS-2, the basic navigation service and augmentation service have [...] been integrated. [...] The augmentation service of only GPS L1 C/A in BDSBAS-1 has been extended to included both GPS L1 C/a and BDS B1I. [...] In BDSBAS-1, the integrity was not included in the augmentation information, but it is included in BDSBAS-2.
BDS-3 (future evolution) extends BDS-2 to global coverage:
The space constellation will consist of five GEO satellites [...] as well as 27 MEO satellites and three IGSO satellites. The current satellites will be a part of the constellation that forms the BeiDou global navigation satellite system.