From “Etiquette”, by Emily Post, 1920:
The bridegroom always has a best man—his brother if he has one, or his best friend. The number of his ushers is in proportion to the size of the church and the number of guests invited. At a house wedding, ushers are often merely “honorary” and he may have many or none—according to the number of his friends.
As ushers and bridesmaids are chosen only from close friends of the bride and groom, it is scarcely necessary to suggest how to word the asking! Usually they are told that they are expected to serve at the time the engagement is announced, or at any time as they happen to meet. If school or college friends who live at a distance are among the number, letters are necessary. Such as, “Mary and I are to be married on the tenth of November, and, of course, you are to be an usher.” Usually he adds: “My dinner is to be on the seventh at eight o’clock at——,” naming the club or restaurant
It is unheard of for a man to refuse—unless a bridegroom, for snobbish reasons, asks some one who is not really a friend at all.