"una cerveza (o más...) para cada una de ellas." This is a general rule in Spanish, that the number refers to each individual in a group. Another example would be the phrase "Everyone who wants pizza, please raise your hands," which in Spanish would usually be "Todos los que quieran pizza, por favor, levántense la mano" -- in singular, because you want everyone to raise one hand. On the other hand, if it's a robbery, you would say "Levántense las manos" because you want them to each raise both hands.

"Ellas beben una cerveza" meaning 'they each have one beer' On the other hand it's common sentences like "Quedaron para tomar una cerveza" But you can drink one, two or more.

It means that each of the girls has one beer for herself.

The former, they each have one beer. If they shared one beer, we'd say "ellas comparten una cerveza" (verbo 'compartir')

Muchas gracias for all your responses. This helps to solve a hotly contested debate on www.duolingo.com