Every participant guild member to the brawls receives some merits, as long as they have entered at least one team in their fray.
If one entered at least 75% of their teams, full merits will be received. There are no differences between guild members, based on how many wins or losses each brought the guild in the brawl when merits are distributed. Everyone will receive the same amount.
If more than 25% of one's battles are flees (teams not entered), the merit rewards are reduced proportionally. If your opponent flees, you get an automatic win.
The number of merits guild members receive is influenced by the level of the Guild Store (level 1 is needed to receive merits).
The other way to earn merits is through ranked battles. Merits are one of the types of rewards that can be found in loot chests, and their number depends on the type of chest (on average, higher number for higher chests).