Can you attack/deal damage an ally?
It sounds as if you were playing something similar to the Star format (though it sounds like you didn't use the color restrictions). The comprehensive rules don't have anything about it, so we have to rely on the group just agreeing on an interpretation. Wizards did list the rules for many popular casual MtG formats in an article written in 2008.

Players sit in a star pattern identical to the one on the back of every Magic: The Gathering card. The player with the white deck takes the first turn and then play continues clockwise around the table.
A color, and the player representing it, wins when its two enemy colors have been eliminated regardless of who eliminated them. This means a player may be able to win even after being eliminated from play! Each player can attack any other player (even their allies), and each player can target any player, permanent, spell, or anything else in the game. Decks are usually built following Standard deck-construction rules, but players can determine for themselves if they'd rather use Extended, Legacy, Vintage, or any other format.

Based on the articles above, it seems like the typical way is no restrictions on targeting or attacking your allies. This makes sense, since your allies aren't your teammates as the rules define teams. I'd say you had every right to attack your ally like you wanted to.