Group A took roughly 4 weeks to sail from Wreathwood Harbor to the port near Holhen Hold on the Basitin Isle.The reason for this length of time was due to them sailing relatively close to the coastline of the mainland instead of sailing directly to Keith's home. Once they arrived, they spent 3 or 4 days completing the storyline there. Once they set sail back out towards Edinmire, it appears they sailed directly to the city instead of keeping close to the coast like they did on the way to the Basitin Isle. This trip, presumably, took 2-3 weeks based off of the distance they looked to have traveled compared to the distance traveled from Wreathwood. It's now been roughly a full day since they docked at Edinmire's port.
During Group A's first 4 weeks, Group B was merely settled at Pinewood. However, on the same day as Keith's trial, Pinewood is attacked by wolves, causing Group B to then go on the run from the Templar. Based on the time it took for 3 people to travel the distance from Pinewood to Wreathwood, even without detours, we can only assume it would take a similar amount of time for 6 people to move a slightly shorter distance, especially with having a couple more days of delay as two of those people were unconscious.
The time it took Eric's ship to cover the distance between the Basitin Isle and Edinmire is roughly the same amount of time it took Trace, Flora, and Keith to travel the distance that it was shown they covered on the world map in the above link Combak posted within first few chapters of Twokinds. In this same time frame, Group B traveled roughly a similar distance (if not longer due to not taking well-traveled roads and paths a majority of the time) as Trace, Flora, and Keith did in a similar time frame. The reason they have yet to reach Edinmire (based on their rate of speed) was due to Zen's attack (and aftermath), and their increased party size, which is double of the original Group A. Now, as for why Reni can be seen in the distance? I suspect that once the shackle finally broke, Natani was zapped back into Zen's subconscious for a brief second and saw that Zen (along with the rest of Group B) was in trouble. After a quick translation of "Oy! My bro is melting his leg off! We have to rescue him! Oh, and Trace's old friends too, I guess...", at the very least Reni went out to where Group B is and will be there within the next couple of pages due to her flight speed due to being a dragon.
tl;dr: The time frame and the location of where Group B is compared to Edinmire checks out. Natani knows Zen is in trouble, and Reni is off to rescue them.