Do you mean that the client only connects to one member to perform all data access? If that's the case you would have to use function calls so that only a single member is targeted.
What is your use case or requirement here? The reason I ask is that using only a single member for data access would, most likely, be sub-optimal. One of GemFire's strengths is that is is data aware allowing it to target the exact members, where the data is stored, that it needs.
--Jens