What other Beaters were there in Sword Art Online? In Episode 8 - Black and White Sword Dance, after Kirito got Asuna to agree to cook the Ragout Rabbit, Asuna dismissed Kuradeel and got refused:


  Kuradeel: I can not leave you alone with a shady character like this
  Asuna: While I can't say anything about his shadiness, he is actually quite strong. In fact, he's 10 levels above you, Kuradeel.
  Kuradeel: Are you implying that I am somehow inferior to him? Wait, don't tell me he is one of those Beaters.


In Episode 2 - Beater, after beating Illfang the Kobold Lord, Kirito was discovered as a Beta player. Reflecting on Diavel's request, Kirito takes an antagonist stance and lies about how far he had progressed during the beta of SAO:


  Kirito: I made it to floors not even the other Beta players could reach.
  Kibaou: Then that makes you worse than a Beta, YOU A GODDAMN FILTHY CHEATER.
  Background Player: He's a Beater!
  Kirito: Beater huh? Yeah, I like the sound of that.


I was under the impression that Kirito was the only "Beater". However, Kuradeel says one of those Beaters, which implies that there are more as apart from him. Using the plural form, Kuradeel is grouping Kirito with others.

So I am wondering, were there any other Beaters in SAO? If there were, do we know who they were?
It would depend on your definition of "Beater." While they are not specifically referred to as "beaters," their actions and hidden ulterior motives would likely label them as such.

If you define it as Beta players who take advantage of their inside knowledge from the closed beta for their own benefit, then there are at least two.


Diavel, attempted to manipulate a certain transaction to ensure that Kirito played a support role during the Floor 1 boss encounter, so he could get the last kill bonus
Coper, tried to use a monster to PK Kirito, so that he could get a certain quest item
Morte, from SAO Progressive, Vol. 2, one of the Beta players that completed the "War of Elves'" quest (one of three people, including Kirito), has a hidden agenda...


Not all Beta players have ulterior motives.


Argo, an information broker that provides the "Strategy Guide by Area". The guide is distributed in NPC shops and provided many resourceful information about the game.


But all in all, the term "Beater" is just a derogatory term non-Beta players use for Beta players that don't share their knowledge and keep information to themselves. Non-Beta players are jealous of the knowledge Beta players have and seek to ostracize them, so the Beta players either have no choice but to share what they know or be forced to play by themselves.