The term which is used to define a non-married partner within a group relationship.
It is standard practice for the freemate to have less responsibility and involvement in the day to day activities of the group relationship. This is largely attributed to the fact that the freemate is likely to have joined an existing relationship. Although, some partners do default into the role of freemate as a result of the two other partners getting married or establishing a deeper romantic connection.
Ashley didn’t feel too bad about staying at home instead of going to dinner with Steve and Shannon; she was the freemate of the group after all.