I am writing to inform you that, after evaluation of the Quidditch trial performances this past weekend, you have been selected for a position on the team reserves as Beater.
You will be expected to attend regular practices with the team, learn the plays, and perform the duties expected of a starting team member. In the event that one of our regular Beaters cannot play on a game day, you or the other reserve Beater will be called upon to fill in. As you have seniority over our other reserve Beater, you will be given priority in the event that this occurs.
On a less formal note, I'm glad you came out for the trial. I'm hardly surprised you made the cut - you clearly have talent, I can't imagine that they've had a good reason for dismissing you in the past. I'd say something about how you'll be the only girl on the team (and, I think, the first in a very long time) and to not let any rude comments get to you, but...I somehow doubt you'd find that an issue.
See you at practice - no way Ravenclaw's coming out of this one with a victory.