Or course copying this awesome, original by Riley, here is one of my own.
My prefix is a strap/loop (or a group)
My first infix is additional
My second infix can mean happiness
My suffix is a cart
This is what I am joining
The bandwagon
strap/loop or group
band
additional
and
happiness
uhh, wag like how dogs wag their tails?
cart
wagon
This is what you're joining
several Riley style riddles :p