A major plot tool of The Last Jedi is:
The Resistance are trying to contact their "allies" in the Outer Rim by transmitting a signal to them through the base on Crait. Unfortunately, there is no response from these "allies."
Who are these allies? Other Resistance groups? Another Imperial faction? The Chiss? Are they revealed by The Last Jedi or any supplemental works?
There were Resistance supporters known since the time of TFA (they were the ones who donated new model X-Wings, for example). From Starwars Databank article for X-70:
The Incom T-70 X-wing is the latest incarnation of a classic design, and the signature combat craft of the Resistance’s Starfighter Corps. Faster than the Alliance-era T-65s, Resistance X-wings are maneuverable enough to engage TIE fighters in dogfights, but powerful enough to take down capital ships. The credit-strapped Resistance relies on X-wings donated or lent by sympathetic New Republic senators and local security forces.
Sure, the Republic center and leadership are gone but the systems those Senators were from, as well as those "local security forces", are still around.
We don't have any canon specifics beyond that, however, until maybe novelization comes out.
There's a description in the film's official novelisation of who Leia was planning to contact after her base on D’Qar was destroyed. Basically no-one of any consequence.
They had to reach out through coded channels to Snap Wexley, Jess Pava, and the other pilots Leia had sent to gather the New Republic’s surviving commanders. They needed to recruit allies from the Outer Rim, contact senators and planetary leaders seeking protection from the First Order, corral military forces left leaderless by the strike at Hosnian Prime, and reactivate C-3PO’s network of droid spies.
In short, the strike at Hosnian prime was pretty effective in killing anyone who could have been of any immediate use. Noting that her messages were received but not acknowledged, the various "Outer Rim" planets she contacted evidently decided that opposing the First Order at that moment would be tantamount to sending their forces to their deaths.
Outer Rim Alliance: Perhaps this group could be the allies Leia spoke of, they literally call themselves the Outer Rim Alliance which is comprised of mainly four factions; Refugees from Artorias led by the Galfridian family & Arbeloa. New Republic Forces from Dibrook led by Admiral Bylsma. Imperial Remnant Forces from Shramar led by Captain Bristow. Along with various pirates, mercenaries & smugglers.
It's likely Lando Calrissian, Maz Katana & the Hutts could be possible allies.
Many planets in the outer rim that were fundamental in aiding the Rebel Alliance would be likely to help again too like; Ryloth, Kashyyyk, Naboo, Sullust, Bespin.
One of major plot holes in whole new SW saga is that there is no organized resistance to First Order except the Resistance. This was "explained" by declaring that First Order destroyed most of New Republic Starfleet in Hosnian system, and that even before that said fleet was reduced in size. At closer scrutiny both of this reasons are not very convincing (fleet would not be so small with First Order openly displaying their strength and it would be likely dispersed trough the Galaxy). Therefore, creators of movie tried to portray things so that Resistance is not only force capable of fighting First Order, but unfortunately for them is only force (at least for now) willing to do so. Therefore, potential allies, although not explicitly stated for now, could be first and foremost remnants of Republic forces, any other planetary forces not willing to submit to First Order, pirates, smugglers, criminals, corporate fleets. With regards to ending of the movie, there is very good chance some of them will appear in next episode.
