Such a blessing is not a sacrament and therefore there is no need for official witnesses (best man or maid of honor). Anyone present can be seated or standing in almost any arrangement since there are no official roles (other than the married couple and the one doing the blessing).
The new Order for Celebrating Matrimony contains an appendix containing “Blessing a Married Couple within Mass on the Anniversary of Marriage.” This includes formulas for a renewal of commitment and a blessing of rings, either the original rings or new ones
