If you mean in Adventure Tour on DS, they are in Blitzland. You need to get through a puzzle of pipes with Mario to reach them.
The answer is Bowser, Eggman, Bowser Jr, Metal Sonic.
you have to beat them (final boss) then bowser Jr. and metal sonic (after beating eggman & bowser and after you complete all their challenges ) then beat eggman and bowser again the they will "follow" you.
from the left, bowser eggman, bowser jr., metal sonic.
Bowser, Bowser Jr., Metal Sonic and Dr. Eggman.
Bowser Jr., Metal Sonic, Bowser and Dr. Eggman.
just click on them but if you are playing it on the nentendo then no cant
You beat all the challenges bowser jr and metal sonic put up and when you finish the missions go beat bowser and eggman again and they will join you.
bowser, egg man, bowser jr, metal sonic its on the back of the sign after the other machine in that town
You use shadow and the order is Bowser, Eggman, Bowser Jr, Metal Sonic the metal gate will the open.
First you hit the Bowser Jr. symbol, then metal Sonic, then Eggman and finally the pumpkin.
I've just finished the adventure mode. Normally there are one for the other rivals and characters, but with Bowser and Eggman; instead there are three matches.Round One: Blazing BobsleighRound Two: Fever HockeyRound Three: Extreme Snowboarding
Once you've completed all missions (doesnt need to be gold star) replay one Eggman and Bowser mission at Blizland to recruit them, for Metal Sonic and Bowser Jr. replay any mission on a green sign they should join you. (Positive about Eggman and Bowser but I finished Avdenture Tours ages ago so I don't remember about Metal Sonic and Bowser Jr.)