Hi,
1. You can send the list of birthday buiddies every month to all with the birthdays mentioned against their name (Please do not mention the year of birth. Date and Month is enough)
2. Along with the list, you can send the horoscope of the star signs (for that particular month) or siginificance of the birthdates ,etc. You can get the datas from google search.
3. You can celebrate the birthday party during the last day of every month. You can provide a big cake for all the buddies and make them cut the cakes. You can also organize for some funny games to make them feel happy. :-P
Attached here are some samples.
Hope it would be useful for you.
Best Wishes.
Bhuvana