If we didn't have enough guildies on at the time- we'd call it for the night. If it was only 1 or 2 people we had to replace, we'd PUG those two slots. Also, people would try and make sure they could be on the whole raid, if that didn't happen we'd call the raid for the night.
Casual guilds aren't raiding guilds- there's a difference. A true casual guild doesn't have someone on standby mode and they don't sit people out.