Since the bondsman need not be a lawyer, it is likely Mr. Miller was a friend, relative, or neighbor of the groom.
I have also seen cases where the bondsman was the employer of a young man who did not have a friend or family member able to agree to post the bond. As the previous explaination stated, in most cases no actual money was ever put up and in most cases the marriage took place and the bond was void. But, back in those days for a man to ask a woman to marry him was considered a binding verbal contract and it he was unable to fullfill the contract becuase he was already married, or other reason, then the jilted bride and her family could sue for damages (to her reputation). So the bondsman had to be someone that the county clerk believed could pay any damages the groom could not.
I have also seen a relative of the bride act as bondsman.