The glass of bubbly is an item included with the Party pack, available at Solomon's General Store for 45 RuneCoins to members or 50 RuneCoins for free players.
If one player with a glass of bubbly uses it on another player who has one, a "toast" animation will be played. Using a glass of bubbly on a player with a Cake hat will give the option to perform the "Congratulation" animation.
Two players toasting.
A player being congratulated.