personally, the only penalty I enforce as a DM for someone missing out is them missing out on all the fun, and a slightly reduced xp gain.

everyone present gets XP for how many people are actually present.

everyone not present gains XP as if the entire party took part.

so if you have 4 people present out of a 6 man party.

the 4 players that show up gain XP as a 4 person party. (larger xp value, given less players) whereas players 5 and 6 gain xp as if everyone had been present (divided by 6 instead of 4)

beyond that, I don't penalize people for not showing up.


except for persistent absences, but that's not really a "punishment" thing, more of a "fill the empty spot" thing.