Based on Hint #1, I came up with the following answer:
TO/TOO/TWO. A phonetic answer, a string of phonemes rather than a string of letters - interesting idea for a riddle!
1.I always let my friends go first.
One goes first, before two.
2.To some of my groups of friends, I mean every[thing].
If you're French, the string of phonemes that is the answer comes out as tout, meaning "every".
3.No matter who I am around, I always have an effect on them.
The word too always has an effect on the other words around it: it emphasises them, in a way.
4.I am strong-willed; none of my friends can change me.
Could again refer to the word too.
5.I have a magnetic personality, so I am always near friends.
None of these words normally appear on their own, but rather surrounded by other words.
All of these words (especially to) are short.
2.I do not like to be alone- I am clingy.
Two things are not alone. Alternatively this could have the same interpretation as Pro #5.
3.I never accept some groups of my friends for who they are, and instead, I try to change them, and always succeed.
This is too again. It changes the words it describes.
4.I'm bossy- usually the one telling my friends when, where, how, why, and under what circumstances things are going to happen.
This could be to, always coming before a verb in the infinitive form.
5.I have 2 personalities- one is almost a teenager, and the other is in his mid twenties
The two letters of "to" are 20 and 15: 15 is a teenager, and 20 is in his twenties. Not mid-twenties though, so maybe there's another explanation for this line!