Shouldn't count unless you're the host AND musical guest. It's not how many times you were ON the show, but hosted it...
Well, considering the genesis of the "Five Timers Club" was an actual sketch on the show that Tom Hanks hosted (and we see it a second time in the Timberlake episode), and that Paul Simon appears in both sketches, explicitly as a member, I would say it does count. Paul Simon has been the musical guest nine times. The "rules" of what does and does not count should get to be made by those who invented the concept.
And Timberlake has hosted outright 5 times; he has been the musical guest in three of those appearances. He also appeared once with N'Sync (technically making that 6 appearances).