I've been in a few guilds lately with varying degrees of this problem.

A month ago I was in a guild that was always just short of having our 25. So we pugged a few spots, took players that were not optimal, or just went short of 25. Progress was slow.

Then that guild merged with another guild that was having the same issue. Progression went through the roof because the best players from each of the old guilds melded into one big butt-kicking machine. But it was like the situation in the article. 15+ people were online and not able to get in the raids. That guild broke up and ended in about the situation it started in.

Now my current guild is one of the top on the server. Most raid nights, there are ~35 people online when the raid is going. Everyone is good so its not like we have to take baddies to keep people from leaving. Nobody raids 100%. Nobody sits 100%. Its a fairly good situation.

Now for my advice. Adjust your recruitment BEFORE you run into problems. You should notice trends. Its unlikely you will go from easily making a raid one week to being five short the next. If you are starting to cut it close, turn up the recruiting. If you are starting to get too many people, slow it down. Just don't wait until you already have a problem. And pay attention to seasonal trends. For instance, you will probably want to preemptively start recruiting just before summer starts because you will see people disappear.