Sources with direct knowledge tell TMZ ... Justin Timberlake and the other 4 boy band members are planning to attend a premiere, of sorts, later this week in L.A. ahead of the animated film's release ... which drops Friday.

We know this coming-together is happening Wednesday somewhere in Hollywood -- but the exact details are being kept under wraps for now. Our sources do say, however, that all of the fellas started making last-minute travel arrangements for this as late as Thursday.

It probably goes without saying ... this is happening at the behest of the studio, Universal, which we're told wants to try and get *NSYNC out in front of reporters and the public any way they can to promote the film ... they obviously know how popular they are as a unit.

Now that the SAG-AFTRA strike is over, just about every major studio is doing this -- namely, getting their big stars together and attempting to get a little momentum amid some mediocre box office returns lately. Brie Larson tried it this past week ... to no avail.

We're told if the strike would've ended a month or so ago ... Universal would've planned a massive premiere with all 5 members, but instead -- they're whipping up this somewhat smaller event in hopes that it can drum up some buzz for 'Trolls Band Together.'

None of the *NSYNC guys have been able to talk about/plug the flick -- obviously JT voice acts in it, but so does Lance Bass, Joey Fatone, JC Chasez and Chris Kirkpatrick. They all have credits on IMDb ... so it seems their work on this goes beyond that nifty new song.

Now, if you think this public outing as a team signals of a larger reunion tour to come -- you're unfortunately wrong ... sources tell us that ain't happening in the near future. Justin's working on a new album/tour, and scheduling remains an issue. Never say never, though.