Everyone's Sims have had a bathroom bottleneck at one time or another.... Here's some ideas to reduce or prevent this!
Put two doors on the bathroom! That way the Sims don't stand in the only doorway and shoo each other with no other way out.
Bathroom Uses You from MATY also helps, but again, 2 doors speed the "throughput."
If you really only want one door, try the Simlogical Bathroom door so the door locks when one Sim enters and unlocks when he/she leaves. Sorry I don't have a link!
You can also put extra toilets in little "closets" and let the Sims wash their hands in the kitchen. Chris's fixed doors should let you make the closets 1x2.
If they're congregating in there to see the new fixtures, nowhatsthis from MATY will prevent that behavior.
If they're in there to talk to the person using the plumbing, I think there's a hack for that too.