Terran Empire: A small but significant number of mirror-universe Terrans have begun to make footholds and alliances here in the prime-universe. Allying with and aiding the Klingon Empire is but one of many tactics employed by Terran command to divide the Federations attention and thin their defenses. Much like the Germans trying to back Japanese and Mexican forces in an effort to keep the United States busy at home in WWII, the Terrans are seeking, for now, to aid the Klingon Empire of THIS universe in keeping the Federation on its toes, and offering better chances at gaining a strong foothold in this universe.
Cardassian True Way: Not satisfied with the direction of their own government, no small number of Cardassian True Way fanatics have turned to outside aid in their ongoing campaign against the threat of non-cardassian influence on Cardassia. Continuing to see the Federation as the great threat to Cardassia, and much like the Terran Empire, True Way fanatics have offered their aid to the Kliongon Empire in exchange for concessions ensuring the solidarity and purity of Cardassia after the Federaton have been pushed out of the sector.
Human Augments: It has been clearly established that Human Augments are working with the Klingons in existing Federation PvE storyline quests.
Jem'Hadar remnant: a rare but growing number of Jem'Hadar have begun to trickle into Klingon space, seeking asylum from the Federation and offering their not-insignificant skills and talents to the Empire's cause.