Except for Frey and Glolov, the rest of the Satan Mercenary Corps all went to Portland.
Frey didn't go because he was on his honeymoon. Gao Yang couldn't bring himself to be so cruel as to drag a man away from his bride on the second day of marriage. Another important reason was that Frey didn't need to switch weapons.
Frey's main weapon was actually a rocket launcher, and grenades were also considered his main armament. As for rifles, Frey had first trained with an M4 and had no trouble transitioning to the HK416, so there was no need for him to change his weapon. He could still use his HK416 just fine.
For Glolov, there was no need to use a custom machine gun, and he could handle any machine gun with no problems adapting to them whatsoever.