Hi Ali... yeah, Lindsey and Steve Ross co-wrote it.
Ross is a guitarist/vocalist who's been a longtime Fleetwood Mac associate, and toured with Lindsey on the 'Cradle' tour.
He also had an instructional "how to play guitar" video out, if I recall correctly (I remember seeing ads for it, which stated that he was a "former Fleetwood Mac guitarist"!).
I just gave "I Want You Back" a listen (I'd forgotten how much I love this song), and these are the lyrics to the best of my ability....
Run around in circles
Like a dog without a home
Since you went and left me
I'm lonely to the bone
I want you back
I want you back again
I want you back
I want you back again
Sittin' right behind me
Is a girl I know
*She put her eyes upon me
*And put me on my toes
I want you back
I want you back again
I want you back
I want you back again
(* I used to think he sang: "She put her hands upon me, and put me on her tongue"... but... uhm... well, I have a feeling he doesn't say that.
)