Stratification is the Answer ...
Victor:
The easiest, quickest approaches to this involve the stratification of your large population, through the creation of intermediate level-groups, to "break down" the too-large group into digestible subgroups. This can be done manually or "automatically."
First, and best, if you wish to precisely / aesthetically control the nature of the groupings, is to manually create levels. An example: Say I have 150,000 members in a "customer" group - I could easily break these into a "sublevel" that stratifies them by alphabetic ranges ("A-F," "G-L," "M-S," etc.) This way, I group them into populations well below the limitation. A possible drawback – I may have to return to “regroup,” if I fail to forehandedly leave enough “room” in the constituent groups for growth, and the “membership” itself reaches the 64,000 limitation.
You can also perform a similar approach through the use of Member Groups (established in a setting within the Dimension Editor, by selecting the level under consideration, then Advanced Properties pane). A dimension level can contain member groups, which are system-generated (the action is "automatic") parents of collections of consecutive dimension members. End users see no difference between member groups and ordinary members, and a "buffer" / intermediate level for drilldown can be provided between a level, say, with few members and one with numerous members.
In either approach, among others, dimension levels (including various procedures at the source data level) can also be used to satisfy the requirement of having no more than 64,000 members under a parent member.
Let us know if this is useful. Good Luck.
Bill