[Legate of Amon Lanc]

Thanks for your addittions, Lommy, radagastly!

I think I have one more thing to add here:
#2

Originally Posted by OF THE RINGS OF POWER AND THE THIRD AGE

But at length the Shadow returned and its power increased; and in that time was first made the Council of the Wise that is called the White Council, and therein were Elrond and Galadriel and Círdan, and other lords of the Eldar, and with them were Mithrandir and Curunír.

This makes clear these members: Saruman, Gandalf, Galadriel, Elrond, Círdan. Although Círdan was surely an important figure, I always considered him to shrink to a picture of the "Old Man and the Sea" after the Last Alliance; nevertheless, the statement that he was a part of the Council makes clear that he must have at least once wandered at least to Rivendell (where the 2851 Council was held, as is said in UT, "Concerning Gandalf, Saruman and the Shire").
I feel sorry for Radagast, because as much as I'd like to see him participating on the Council, the quote above suggests that no other wizards were in the Council. Since the Council does not equal to "The Wise" ("The Wise" is a more general group of Istari and Eldar), I think Radagast is out.

Glorfindel. I am not sure, radagastly, where you get that information that the fact that he was sent back has any connection with the Council - there surely were many who could have been in the Council and were not.
But I agree with putting Glorfindel on the list. The black horse, eh? (wait, his horse was white) Maybe he's not the outsider after all. In my opinion, from what we know, we have more evidence pointing that he might have been a member than, let's say, Celeborn. First, he was a Noldo, and a very powerful one, indeed. Of course, as we know, in Tolkien's world, power always does not mean being fit for something. But he was certainly active at least on the Council of Elrond. And from there I'd serve with one quote:

'Then,' said Glorfindel, 'let us cast it into the deeps, and so make the lies of Saruman come true. For it is clear now that even at the Council his feet were already on a crooked path. He knew that the Ring was not lost for ever, but wished us to think so; for he began to lust for it for himself. Yet oft in lies truth is hidden: in the Sea it would be safe.'

We might notice some interesting things in Glorfindel's speech here. I know my assumptions are based on mere words, but that's the best we have thus far. Anyway, you must judge it for yourselves. The first thing is, it seems to me that Glorfindel speaks about the (White) Council in a very familiar tone. Second, more important, that he says "Council" in one sentence, and right after that he says "he wished us to think so". This may mean "us good guys", but I don't think it was spoken openly of the Ring before the council of Elrond, so if Glorfindel weren't present when Saruman spoke about the Ring (i.e. on the Council), he impressively quickly adapted himself to the position of "us" from the Council. This indicates to me that he actually knew about Saruman's speech to the Council before, implying that he was there when it was spoken, thus, being a part of the Council.

Thranduil&co. I don't think Thranduil was there. As radagastly said, he is never mentioned to have anything with the Council (and at minimum in the year of Dol Guldur assault he prepared for the Battle of the Five Armies and didn't seem to care of any Necromancers). Also, in the quote above it is said that there were "other lords of the Eldar". Eldar, at least in Third Age (of which we speak), meant only Vanyar, Noldor and Teleri. Thranduil, although being of Sindarin heritage, was not a lord of Eldar, but lord of Avari. This would, I think, leave all Thranduils out.

Galdor. I am very reluctant about this one, although radagastly implies he might have been there. He was sent to the Council of Elrond "as an errand from Círdan", something like his deputee, since Círdan couldn't have come personally. In my opinion, he's not doing exceptionally good job (in comparision to, let's say, Erestor - see below). If Galdor was a part of the Council, then he must have a very feeble mind, because he seems to be totally "out" in some quite obvious things. And he adresses the others "the Wise"; therefore, apparently not counting himself as one of them (although, the Wise does not equal the Council; but I think all of the Council were Wise, but not all Wise were in the Council, not vice versa):

The Wise may have good reason to believe that the halfling's trove is indeed the Great Ring of long debate, unlikely though that may seem to those who know less. But may we not hear the proofs?

So in my opinion, not Galdor, he's a mere emissary of Círdan.

Erestor, on the other hand, is something different. He is the "foremost of Elrond's counsellors", and he has some quite good addittions to the topic during the Council of Elrond. He is really active and (in contrary to Galdor) seems to know what he's speaking about, though there is no real evidence of him being in the White Council. But since we know the 2851 Council took place in Rivendell, it is possible to imagine him there (as well as Glorfindel).

The last is the most problematic. Celeborn. Well, this is hard. The fact that Galadriel is in the Council (even summoning it first) does not necessarily mean her mate was there as well. I'm waiting if anyone to support or disqualify him. So far, the best evidence of him being in the Council is that he is an "Eldar lord" (and we don't have too many Eldar lords here). But the fact that Galadriel in Lórien speaks to the Fellowship about her first summoning of the Council, her mate is sitting right beside her and she does not mention anything like "...and Lord Celeborn was there too..." speaks against him for me.