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When I first read through the Tomb of Annihilation I thought that it stated that the guides give you advantage on Wisdom (Survival) checks made to navigate through the jungles of Chult. However, now that I am going back through the book, I can find no mention of this.
What advantage, if any, does hiring one of the guides grant to ability checks made to navigate the jungle?
From "working together" in the Basic Rules (p. 59) you can apply advantage without resorting to anything "custom" to the adventure.
Working Together
The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters.
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another
One way to tailor this to the published adventure is that whomever has the highest Wisdom modifier tries to navigate, and the guide helps. Or, because they are not familiar with Chult, you make a group check (basic rules, p. 59) with advantage because the guide helps.
Caveat: most of Chult is unexplored and poorly mapped. Most of the time, you can expect there to be some uncertainty, as described on pages 38 and 39 of the book, sections on the Land of Chult and Navigation. There are some NPC's whose back story includes being very familiar with a particular area, however, in the interest of avoiding spoilers, I'll get no more specific than that.
If the party has a Cleric or Duid, the guidance cantrip is a handy way to increase the chance that the party Navigator, be it Guide or PC, has an increased chance to not get lost/head in the right direction.
The guides do not necessarily interact, mechanically, with the navigation checks described in "The Land of Chult." But there are two ways that they do/can interact with navigating, generally, the jungle:
They may have a better Wisdom (Survival) modifier than all of your party members. In that case this GM finds it completely reasonable to let the guide do the "navigating," too, and use that modifier. (In my current ToA party that's not the case, but during "interviews" the topic came up.)
The guides (might) know where they're going! If you decide to hire Azaka and go to Firefinger or Orolunga, for example, she knows where those are. I.e. she can put her finger on the right hex of Syndra's map and say "it's here--let's go." Making progress toward the relevant hex is still governed by the navigation rules, but the desired destination is known. If your party hears rumors of Firefinger or Orolunga and wants to go there, but has no guide who knows that location... there's a lot of empty hexes they have to check.
Page 38 of ToA says, under the "Navigation" heading:
Have the players designate one party member as the navigator. The navigator might be an NPC, such as a guide, and the party can switch its navigator day to day.
At the start of each new travel day, the DM makes a Wisdom (Survival) check on behalf of the navigator. The result of the check determines whether or not the party becomes lost over the course of the day. [...]
If the check succeeds, the navigator knows exactly where the party is on the players’ map of Chult throughout the day.
If the check fails, the party becomes lost.
Many of the guides even get special bonuses to their Survival skill.
