After your individual score is shown and you receive individual reward drops (which are similar to what you can get from Free Lives), you are brought the SBL results screen, where the Group Rank and completed Group Goals are announced, and the MVP Titles are awarded. At the end, you will receive SBL medals for each of these, which you can exchange for past Event cards and various items in the Trade shop.

The Group Rank grants blue boxes, which are small drops containing 10 to 15 medals each. See Group Rank for the exact number of drops for each rank.

The Group Goals drops red boxes, which are slightly bigger with 35 to 55 medals each. See Group Goals for the amount of boxes each goal rewards the room with.

Finally, the MVP Titles give big medal bonuses: receiving a silver title for winning one of the categories is worth 1500 medals, and the gold title for the Overall MVP comes with a 3000 medal prize - far bigger rewards than the blue and red boxes.

Ranking Rewards are distributed at the end of each day and after the event ends, and are probably the biggest source of medals. The daily ranking is based on the total highest Voltage you got on that day for each of the available songs, while the total ranking is the sum of all the daily ranking Voltages for every day of the event. Making sure that you play every song at least once per day should ensure you get a big bunch of medals.

Additionally, there are also rewards for reaching a total amount of SBL lobbies played during each event, which can be collected by just playing. There are both Individual rewards per player and Overall rewards resulting in extra medals for all players.

Check the Rewards listing in-game for details on ranking tiers, play count thresholds and rewards for them.