The organization has the foundation as set forth by 1-2e, i.e. as one increases in level/power there are fewer druids at the "top". The 13 MOST powerful druids of the world constitute the "governing body" of the organization, "the Grove."
The Grove oversees various sacred sites and the security/maintenance of places of natural power...usually through the assignation of middling to high level druids assigned to those sites, called "Keepers."
I should clarify (myself, haha). As far as I have "figured this out", one does not become a "Keeper" before 7th level. And just being 7th level or higher does not guarantee the Grove will make one a Keeper (i.e. PC druids need not be tied to a particular area for the rest of their lives. All members of the organization have a role to play...and some [PCs] are assigned/do roam the realms to be able to gather information and patrol the risings and fallings of evil and good and law and chaos).
The members of the Grove are all over 10th level...if I had to stat them out, I'd even say they are probably all 13th or better. I think the original term was Hierophant (sp?).
There is one Grove member for each of the original sacred trees of druidism (a Lord or Lady of Ash, Lord or Lady of Birch, Rowan, etc...). But that does not mean that there are only 1 of that level druid or two druids within the Grove could not be the same level.
Yes, once you get above 7th level there are fewer and fewer. But that is more of a fact of how I have high level classed individuals set up in the world. Above 7th level, there are fewer wizards, fewer "great lords [fighters] and high priests, etc.
A druid who wishes to take a place within "the Grove" must usurp another through a battle of abilities (spells, shapeshifting, etc...Exhibit their surpremacy in their understanding of and union with the Balance, basically.) The defeated druid is not, necessarily, nor must be slain...simply defeated.
The "Kill 'em take their place" thing for high level druids in 1e doesn't really exist. The only druid that there is ONE [level] of is the Lord/Lady (currently, it is a Lord

) of Oaks.
It should be noted that I have not yet had a PC druid actually achieve such a level. So it's purely always been an NPC/fluff/story element thing.
Insofar as I have had to use this in actual play...well, I haven't....so I have given no thought or concern to the idea that there are or could be very high level druids floating around who are
not in the Grove.
But, just because they are a worldwide organization does not mean there are large numbers of druids greater than 7th or 8th level floating around.
I suppose, if I had to make a story reason, 10th or higher level druids who are not in the Grove simply are not on the Orean (physical/Prime) plane. They spend their time expanding their consciousnesses. Spend a few decades as a tree or a badger or cloud or whatever, visiting/experiencing/assuming the form of beings and creatures on the other planes of existence with which the druids are concerned (the elemental planes, basically), "communing with nature" (in some kind of solitary trance, hermit lifestyle, or some such), etc...
So, again, in my world druids are not really out for personal power or worldly "achievement" in a fashion that they would want to be reincarnated repeatedly through magical means.
S'all about growing the soul...expanding the consciousness...ya feel that, man. <snaps fingers and hugs a tree>
--SD