We did this with my guild. We left and formed our own new roster (12 people of 20), took us 3 weeks of poaching falling apart guilds and actively recruiting but we have a 25 man roster for 3 days a week with 5 reliable back up raiders who can only make select nights. I'd say we get 2-3 people from each guild that's fallen apart.
My advice for people who want to follow suit.
1) be active. You can't passively recruit anymore, if you're < 6/10 nobody will apply directly to you.
2) stalk the wow forums, Reddit posts, wow progress for your server.
3) in game mail them with all officer btags, or add their btag if you have it.
This is super exhausting if you're doing this alone so I made my guild know the issue and asked them to see if friends guilds had broken up. When you see an app, look at their prior guild on wow progress. See if they've had people leave, and if so start to pm people to cop a feel for who is LF a guild. Lastly, try to split it up evenly. For example, I handle Reddit posts via PMs from the subs. One guy handles wow progress, another handles wow forums and all our non officers just pm the people my btag.
Best of luck.