You can become co-breeder at any time ! Before or after the mare is in foal.
The deadline is the birth of the foal ! Because we need to do the birth certificate, no later than 10 days after the foal's birth.
Once this date past, we cannot add any more co-breeder.
But no need to worry, if you like a project on https://www.myhorsefamily.com/ you have usually 11 months to take a part and become a co-breeder ! :)