The Beta is second-in-command to an Alpha in a clique. Whenever the alpha's not around, the beta's the temporary alpha. Although if anything happens to the alpha where they can't be the alpha anymore (like if they get kicked out of the clique by the others members or something), the beta becomes the new official alpha.
Alicia had actually tried a number of different times to be the alpha of her own clique, but it never worked out. Before Massie and her family moved to England, Alicia finally accepted the fact that she's better as a Beta than as an Alpha, and labels herself a BFL (Beta For Life). Although she was happy when she was declared the new official alpha of the Pretty Committee in A Tale of Two Pretties, she still saw Massie as the true alpha.