Could be a thread by itself....
... Gary Barlow (Take That)
Noel Gallagher (Oasis, but apparently not until 2005!)
Cathy Dennis (Kylie Minogue & S Club 7 - 2001)
Edit: Just had a look at the pure numbers... you know, since about 2010, apparently every song has had at least 10 songwriters, which is why people like David Guetta might technically count - but, in my opinion, shouldn't really.
Eminem in 2002 should count though.