When people trying to find a party for games mods like brawl/deathmatch and finally when they find it, at the end of the match all leaving and you need to start again.
Often happens when you playing in a current group and after a match, they leave, in the second wave of searching you facing with same players which were before.

So my suggestion is to trigger a system on low population mods which will give you bonus exp/more loot/troops to deploy for each match you playing and staying in the same team without leaving it.
For example, I'm playing a brawl on low population, after finding enough players for the match and at the end of the match will be accrued bonus (exp/more loot/troops to deploy) only for players who didn't leave the party (on both sides). Each match will accrue more bonus and after, maybe 3 matches it will reset.
Maybe in this way players will not immediately leave and will remain some time in the current group.