Network operator typically configures these pools. Since a MME can be part of single MME, the network operator will carefully configure these MME pool areas, as to load balance the serving UEs. Another way of configuring pools is by using the DNS mechanism.
However MME Pool areas can overlap because, of the TA they serve. As we know each MME can serve a set of TA i.e. TA list. But MME can never be shared among multiple MME pools. [MME is identified by a unique MME Code and the group [MMEGID] to which it belongs to, globally. Also the PLMN ID is associated with it to form GUMMEI ] It is just that the TAreas may change for an UE. And if the UE roams between them, the MME which has that respective TA in the correspodning pool area [geographically] in its serving list will be chosen to serve the UE.