In despite of the seemingly single-planar nature of many sects, here are some suggestions by Greg and myself for secondary planes of influence for some of the myriad sects populating the Planescape multiverse.

Converts are prominent on the Outlands, where all the factions meet eventually, and they can travel easily to their latest factional area of choice.

Dispossessed: Their primary plane is Pandemonium and Carceri is a close second. The Dispossessed fit in very neatly (so to speak) with the cruel, gnawing jealousies of the Carcerians.

Ring-Givers: The Planes of Chaos mentioned this sect picking up in the Abyss. They also have a presence in Sigil, and of course they travel to Gehenna to bug the resident Fated.

Mathemeticians: This sect's beliefs are not much use outside of their primary plane (Mechanus). The cubes and other geometric shapes of Acheron would have proved an interesting plane of study for them, and a subsect (!) called the Geometers now exists to chart the shapes of Acheron. They're presently looking for a way to examine the shapes of Ocanthine plates.

Order of the Planes Militant: Bytopia, although mainly to filch land for Mt. Celestia. The presence of the Harmonium on Arcadia keeps that from being a good secondary plane. Planes of Conflict discusses this in some detail.

Verdant Guild: The Verdant Guild were amongst the first in recent times to hear of the wild and untamed wilderness plane of Pangaea, and are trying to raise awareness of, and protection for, this unspoilt paradise, as they see it. Others wonder why they're so keen to defend a load of shrubs, and suggest that the Wylders are hiding something. Wylders can also be found on Arborea and in the Wild Wood of the Hinterlands.

Guardians: The Guardians can be found on the Prime as often as not, protecting the innocent Clueless (as they see it) from evil otherplanar influences. Many would call it meddling, and Clarion the Guardian has been seen arguing strongly with Cirily the Planarist about helping primes.

Merkhants: While they would no doubt love to have more influence in Bytopia, the greedy nature of the Markhants makes them more fit for Gehenna, the plane of Greed. The Fated there get along with them as well as with anybody.

Ragers: Their primary plane is probably Limbo, and their secondary plane is Ysgard, although other Ysgardians are keen to point out that brainless fighting is for Acherians.

Communals: Acheron is a good plane for those who have taken their communal conformity to an extreme. Baator also intrigues those who believe a structured government can aid the people (or claim to. It's more likely they want to be the first among equals).