Third try.... Are you:
Cells?
Hello there! How do you do?
What am I? Oh, have no clue?,
OP stated this is just a filler.
but I still reside in you!
Our body is comprised of cells. They reside within us, by making us.
Let's just say I am your glue.
Cells make up everything, including tissues that hold us together.
I'm not alone, let's say "a crew",
A single cell can't do much, but with a "crew", cells make up proteins, tissues, organs, etc.
read my name and you will "whoo",
There are so many different types of cells. Not sure what makes it fit, but I'm sure something very clever could fit here.
my size is small, I'll be true,
Cells are the building blocks of life, and generally we discuss "single cell" as being the smallest living thing.
but I'm the longest, I bet you knew.
If we were to line up all of our cells in a single row, they would be incredibly long.
Second try. Are you:
The genome?
Hello there! How do you do?
What am I? Oh, have no clue?,
The human genome is so incredibly long and complex that we haven't fully discovered all that it can do.
but I still reside in you!
Every cell in our body contains our full genome.
Let's just say I am your glue.
The genome is what makes us who we are and "holds us together" as it were.
I'm not alone, let's say "a crew",
Made up of DNA (which is made up of 4 bases), that is organized into 23 Chromosomes, and genes within the chromosomes.
read my name and you will "whoo",
Genome -> "Ge -know - me". Leading to a, "I know who?" Or, potentially thinking of the names of genes, Deoxyribonucleic acid, Ribonucleic acid, and other fun names in the genome.
my size is small, I'll be true,
but I'm the longest, I bet you knew.
The human genome is incredibly long. Though, interestingly, it is longer in some plant life (grain) than it is in humans.
Previous attempt
You are:
DNA
Hello there! How do you do?
What am I? Oh, have no clue?,
We are still discovering some of the mysteries of DNA
but I still reside in you!
DNA is contained within every cell in your body.
Let's just say I am your glue.
I'm not alone, let's say "a crew",
DNA is made up of 4 bases that link together, and make us who we are.
read my name and you will "whoo",
Deoxyribonucleic acid is a very long name that makes most people go "huh?"
my size is small, I'll be true,
but with my length you wouldn't screw.
DNA is very, very small, but is in a double helix form, which winds around like a screw.