Oak Ridge Boy Richard Sterban told The Nashville Tennessean that Bush had requested the group sing "Amazing Grace" at his funeral.
The group had been friends with Bush since they played at a White House barbecue when Bush was vice president.
He told them they were his favorite country group.
Oak Ridge Boy Duane Allen says he distributed flyers for Bush in the 1960s when Bush was first running for office in Texas.
At Thursday's service, they said, "For decades we have sang for him, and this is, again, a real honor to be here. What a lot of people may not know is he fancied himself to be a good bass singer. He was not." Their comment drew a lot of laughs. "We'll sing for our President."