There are several roles in the brit milah ceremony where family members and friends can participate. It is considered an honor and a privilege to be asked to fulfill these roles.
Sandek
Being named the sandek is the highest honor and most prestigious role that a family member or friend can be given in the brit milah ceremony. In traditional ceremonies, the Sandek also holds the baby while the mohel does the actual circumcision. But not liking moving targets while doing surgery, most contemporary mohels will place the baby on a plastic restraint board of the kind used in hospitals for the performance of the circumcision.
The honor of being a sandek can be given to any family member or friend the parents so choose. Most commonly the honor is given to one or both of the grandfathers (there can be more than one sandek and they could be grandmothers as well). If there are two sandeks, they sit side-by-side and either alternate in holding the baby or one holds the baby during the ceremony and the other comforts the baby with wine on a pacifier during the circumcision.
There is a tradition that the sandek should be a person worthy of respect, one the parents would have their child look up to. While it is no means required for the Sandek to wear a tallit and yarmulke during the ceremony, both are encouraged and lend elegance and spirituality to the occasion. The sandek, as he is holding the baby, is the only one who sits during the brit milah ceremony. Everybody else stands (if they are able) in honor of the newborn.
The Kvatter and Kvatterin.
The role of the kvatter or kvatterin (female kvatter) is to bring the baby from the mother to the sandek by way of the Chair of Elijah (see section on “Chair of Elijah“). These can be any one or more people of the couple’s choosing. Traditionally, a young couple seeking to begin a family themselves is chosen for this role. But in contemporary brit milah ceremonies anyone—relative or friend–can share in this honored task. Often multiple family members will form a human chain of kvetters to pass the baby from the mother to the sandek. Formerly, being a kvatterin was the only role a woman could play at a bris ceremony; now, of course, women can share any of the honors of the brit milah service. Also, while the sandek does have to be Jewish, the kvetter/kvetterin do not.
While the above are the two official honors at a brit milah ceremony, there are other means of allowing relatives and friends the privilege of participating in the event:
Lighting of the candles. As mentioned previously, while this is not a mandatory part of the brit milah ceremony, it does lend beauty and dignity to the event. Any guest present can light the candles and say the blessing over them. However, lighting of the candles cannot be performed on Shabbat.
Blessing over the challah. Saying the blessing over bread before starting the festive meal is a time-honored custom and brings a sense of increased religiosity to the brit milah ceremony.