A contemporary group is a group of animals that have had an equal opportunity to perform - same sex, managed alike, and exposed to the same environmental conditions and feed resources. Contemporary groups account for environmental effects, so that the remaining differences among animals are due to genetics. They are a basic part of EPD calculations.
A weaning contemporary group might be bulls born within a 90-day period and weaned within a three-day window. Bulls and heifers are in separate contemporary groups, and things like management codes and group codes may further split them into more groups. Data for a contemporary group needs to be submitted to the breed association at the same time, otherwise bulls will be in different groups.
For Angus bulls, the contemporary group is defined at weaning. The number in the group will never increase, although it can be split into more groups as animals are separated. ET and non-ET calves will be in separate contemporary groups, even if they are weaned at the same time.
If you send only one bull to the IBEP test, that bull is now a contemporary group of one for ultrasound and yearling data, so that data will not be used in EPD calculations. In contrast, if you send two more more bulls from the same weaning group to the IBEP test, they will make up a contemporary group for ultrasound and yearling data, and that data will be used in EPD calculations.
The general concept of contemporary groups will be the same for every breed. Contemporary groups are initially defined by the owner of the bulls. However, each breed association may handle the groupings differently, for example the way the initial group may be split into smaller groups based on various management codes. If you have specific questions about how your breed defines groups, contact your breed association.