OLAC uses the OAI-PMH protocol to harvest metadata for language resources from participating archives. Since each application built with the clld framework comes with an implementation of the data provider side of this protocol, registration of a new archive is simple. By default, the details pages of each language identified with an ISO 639-3 code will be disseminated as language resources.
Now on to the bragging: As can be seen on OLAC's archive metrics page :
It has to be noted, though, that OLAC only indexes resources for languages identified by ISO 639-3 codes; thus it's difficult to register the resources provided by APiCS Online or eWAVE. Hopefully someday it will be possible to also register resources tagged with a Glottocode (Glottolog's languoid identifier).