I assume you're talking about in-game and not MOBAFire. You'll have better luck in the General Discussion forum, this Support topic is only for MOBAFire.com support so it doesn't get a lot of traffic. Moving topic.
If you added them in your last 10 games, you can look yourself up on lolking.net and try to find their name. Other from that, that's all you can do without getting a Rioter involved =(
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Any info much appreciated.