I seem to recall that you just ordered multiple construction frigates to work on the target structure, but I simply don't use this upgrade, so it's not a fresh memory at all.

Unless your build speed is set to "slow", communal labour is completely unnecessary.  The problem is by the time you get can a second one of those finicky little construction frigates into position, the first one has already finished the job.  A second issue is that new colonies start with only one construction frigate, so only semi-mature colonies actually have multiple construction frigates to do this.  All things coming together, build times are largely negligible in this game and it's moving units into position to build the structures where the inefficiencies crop up.

As I said, build times aren't that big a deal.  I don't consider it a major disadvantage for Advent at all.  The real disadvantage for Advent is that they don't have any good economic techs in the 4-7 lab level.  As a result, mid-game economic booming is a real dead-zone for Advent.  Their only real option in that stage of the game is to trade port spam, something the other factions can both do just as well if not better. 

That said, Advent has the most solid military overall.  TEC starts strong and kinda dwindles as the game goes on.  Vasari starts weak and gets stronger and stronger until they're practically unbeatable in the end-game.  Advent starts strong and ends strong, very reliable units that are great from the first minute to the last minute of the game.

Not saying you'd fill every logistics spot.  If you colonized effectively in the early game, you could have 15 or so planets each with room for about 5 trade ports.  That's a lot of logistic space, and even if you go crazy you won't fill it.  All I mean by trade port spam is that you build a lot of trade ports.

If your empire is small (8 or fewer planets) there's usually no reason to go above the 3-lab civic level anyways since the payoff is too small.

Uh, you only get one construction frigate from a freshly colonized planet, so no, you can't use communal labour in these instances.

When you have 3-4 construction frigates, the work goes pretty damned fast... but... the Vasari upgrade makes it look like a snail, by the end of the tech trees Vasari can damned near insta-populate a grav-well with structures.

Also, there is no need to manually select the frigates and tell them to work together, the auto-cast should be functioning. Sometimes if you queue up more buildings than you have construction frigates - the frigates will do 1:1 instead of trying to 3:1 each structure.

Double check what's happening... if your auto-cast is truly failing, you need to report it as a bug. When it's working you'll see several construction ships spewing little purple waves of (nanobots?) something at whatever the structure is they are putting up. Come to think of it, I'm not even sure there is an icon just for the ability to autocast... I think you may just have to look out for several ships working on one thing - but I rarely play Advent, so who knows... Point in case though, it should be working.

If you are having any doubts about Advent, just roll up a couple of Guardians with repulse, and watch 2 ships w/ some LRM backup rickroll TEC/Vasari fleets because they can't get close enough to hurt you.