Combining selected text into a long string of text

I have a worksheet with a column of names and a column birthdays.
The birthdays is in text "yyyy.mm.dd"

I would like to figure out who should I held birthday party for in each month
so I would like to generate a column displaying the month they were born, it needs to read the text and make it into number. for example "2011.02.21" into "2"

Then I would like a function that look through the column of month and generate a large string of text with the names of people who was born in a particular month seperated by a comma, ex: "John Doe, Jane Doe, ......."

Is there a limit for the length of the string? There are around 100 people in the list, I doubt the stirng would go over 20 names.

Can anyone help me with that