Summoned how?

[Kirinki54]

'I have thirty with me,' said Halbarad. 'That is all of our kindred that could be gathered in haste; but the brethren Elladan and Elrohir have ridden with us, desiring to go to the war. We rode as swiftly as we might when your summons came.' 'But I did not summon you,' said Aragorn, 'save only in wish. My thoughts
have often turned to you, and seldom more than tonight; yet I have sent no word. (The Passing of the Grey Company)

How were they summoned then?

[HOBBIT]

Too lazy to go back and check, but weren't they summoned by Galadriel?

[easterlinge]

Presumably the Giant Eagle had something to do with it.

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