A dispensing cap system for dispensing a material through a bottle neck opening and into the bottle to mix with contents of the bottle, comprising: a cap member having a web portion and a connection structure for non-threading engagement with the bottle neck; a seal member having a seal surface arranged for sealing contact with the bottle neck, the seal surface being sufficient to provide sealing contact with bottle necks of different opening sizes; a storage member assembled to the cap member and having a storage volume for holding material; a valve member having one or more valve ports and one or more dispensing ports, the valve member configured for sliding movement between a storage position in which the storage volume is sealed and a dispensing position in which one or more dispensing ports in the cap member are open to allow the material to pass through from the storage volume to mix with contents of the bottle; a valve nipple configured to fit onto the valve member for sliding movement between a valve closed position and an opened position in which contents of the bottle are allowed to pass through the valve member for consumption by a user; said connection structure comprising a set of finger portions depending downwardly from the web portion and having distal ends, the finger portions having engagement portions to engage a neck flange of a bottle inwardly of the finger portions, the cap member having sufficient resiliency so that the finger portions can splay outwardly to allow the finger portions to pass over a neck flange as the cap member is installed onto the neck and to allow the engagement portions to engage the flange to secure the cap member onto the bottle neck, wherein there is also a rigid member attached to the seal member for engaging a lower portion of the valve member when the valve member is in the dispensing position for holding the valve member in the dispensing position. a cap member having a web portion and a connection structure for non-threading engagement with the bottle neck; a seal member having a seal surface arranged for sealing contact with the bottle neck, the seal surface being sufficient to provide sealing contact with bottle necks of different opening sizes; a storage member assembled to the cap member and having a storage volume for holding material; a valve member having one or more valve ports and one or more dispensing ports, the valve member configured for sliding movement between a storage position in which the storage volume is sealed and a dispensing position in which one or more dispensing ports in the cap member are open to allow the material to pass through from the storage volume to mix with contents of the bottle; a valve nipple configured to fit onto the valve member for sliding movement between a valve closed position and an opened position in which contents of the bottle are allowed to pass through the valve member for consumption by a user; said connection structure comprising a set of finger portions depending downwardly from the web portion and having distal ends, the finger portions having engagement portions to engage a neck flange of a bottle inwardly of the finger portions, the cap member having sufficient resiliency so that the finger portions can splay outwardly to allow the finger portions to pass over a neck flange as the cap member is installed onto the neck and to allow the engagement portions to engage the flange to secure the cap member onto the bottle neck, wherein there is also a rigid member attached to the seal member for engaging a lower portion of the valve member when the valve member is in the dispensing position for holding the valve member in the dispensing position.