Why do Mr & Mrs. Eakin receive a message from Lacuna that Clementine erased her memory in Eternal Sunshine?

Rob and Carrie are the ones who invited Joel (Carrie) to the party in Montauk, where he meets Clementine for the first time (we see in a flashback as his memory is being erased). As good friends, a "couple" type friends, the ones who essentially set them up, that would be someone to notify.

Joel: My name is Joel Barish, and I'm here to erase Clementine Kruczynski.

Doctor: Very good. Now, tell me about, uh, Clementine.

Joel: Uh, I was... living with this woman, Naomi, uh, a couple years ago,
and my friends Rob and Carrie invited us to this party at the beach... I don't like parties.
Naomi couldn't go, but I went... and I, uh, met Clementine.

Not seen outright, but couples normally hang out with other couples, so it's likely that they had double dates or otherwise hung out.

At the beginning they didn't know her:

Carrie: I saw you talking to someone pretty!
Rob: Yeah, man, who was that?
Joel: She was... just a girl.

At another point, after Clem gets the memory erased, we hear Carrie talking about Clementine in a familiar fashion:

Carrie: What can I say, Joel? You know Clementine. She's like that. She's ... impulsive. She decided to erase you almost as a lark.

So at the very least, close acquaintances if not outright friends at that point. Close enough to know her character traits. So that's why they got the card.