Congratulations that you finally found Opeth, sir! One of the best bands in the world in my book and i'm a fan of theirs for almost their whole career now.
And yes, Mike is an absolute mastermind and seems to be able to write songs like that on his own. There are some of those genius-dudes out there and I honestly also don't get HOW they do it! Probably you just need to go for it and practice. There's a saying about young photographers that goes like this "You first 10.000 photos are your worst" or something. I think the message behind it also applies to music or songs... so move on, get yourself up and try it! Don't be afraid to "fail", because with every mistake you learn lots of stuff!
that being said let me introduce you to my band EFFLORESCE. Since we all love Opeth we probably do sound a bit like them. Actually that's a comparison, that keeps coming back and someone even said in a review of our latest album "If someone wonders how Opeth would sound like with Anneke Van Giersbergen or Simone Simons on vocals, well this album is exactly the answer". So yeah...
But getting back to your initial question: We do write songs completely as a group! In our rehearsal room, everyone is throwing ideas back and forth and there's a lot of trial & error involved, but somehow it works for us and everyone feels involved of course, which is great. Feel free to check us out on www.effloresceonline.com
And keep on listening to Opeth and come up with your own stuff, you can do it, man! I'm sure about that!