There's still time to join for Ashley Parker Angel to join in on the reunion fun.
Sources close to the band tell TMZ, three of the four members are recording their tracks in L.A. -- but Jacob Underwood (second from right) lives in Nashville ... so he's submitting his parts from across the country.
We're told Jacob's coming out to L.A. for his birthday in April -- the first time the entire group will have been in the same room in SEVEN YEARS.
So far, no performances have been scheduled -- but according to sources, it's only a matter of time once they finish recording.
Erik expressed his disappointment in L.A. yesterday -- telling us, if Ashley really wanted to rejoin O-Town ... he would have figured out a way to "multi-task" the reunion and his acting career.
Erik says he and the rest of the band have all come to terms with Ashley's rejection -- but explains, "I would just think he would want to come on board and be a part of it ... but he doesn't and that's cool."
In a statement to TMZ, Ashley explains, "O-Town was one of the greatest chapters of my life, so when the idea of a reunion was brought to me, of course I was intrigued."
He continues, "However I have made the decision not to be a part of an O-Town reunion. It was a difficult decision, but ultimately necessary to move on with the next chapter of my career."
The rest of the band has already gotten back together -- and is planning an upcoming concert tour sans Parker-Angel.
As for Ashley, dude just finished a stint in "Hairspray" on Broadway and his rep tells us he has a feature film coming out later this year.
Sources close to the band tell TMZ ... Erik-Michael, Dan, Trevor and Jacob are back in the studio ... sans one member who just didn't feel like getting the band back together -- Ashley Parker-Angel.
We're told the guys -- who were all plucked out of obscurity on the very first "Making the Band" in 2000 -- have been kicking around the idea of a reunion for a while ... and have spent almost a year trying to get Ashley on board.
But a few weeks ago -- when it became clear Ashley wasn't going to change his mind -- the guys decided to officially move forward without him ... and are now trying to work out some tour dates.
They want it all ... or nothing at all.