Friend invited someone else to our 3-weeks Central Asia trip, not sure if I should say yes

One of my best friends, Emma, and I (23F) have been planning a 3-week road trip in Central Asia (already mid June) since a few weeks. We’ve traveled together before and Emma is a great travel buddy.

Today she offered another friend of hers, Ann, to join us as Ann doesnt have any summer plans yet (didnt ask me about it in beforehand). They recently traveled together in Japan and apparently it was great. I’ve met Ann before (same high school), but we don’t really know each other.

I’m quite easygoing and get along with most people and Ann seems nice. It would also make the trip cheaper and would be cool to get to know someone from my hometown better. But Im a bit unsure about the group dynamic. Emma can be quite a type A personality and tends to talk a lot, which I don’t mind at all when its just us (or also when meeting people while traveling it was no problem), but in groups of 3–4 friends I’ve sometimes found it a bit much. Im also a bit afraid that I would feel like the 3rd wheel (even though Emma and I have been friends since we are kids). Since we are doing a road trip for 3 weeks, there’s basically no option to split up if it doesnt work out.

Emma said Ann would totally understand if I’m not comfortable, but I still feel a bad if I say no. Also it’s not totally clear if Ann can 100% join, she would only fully check her availability if I say yes.

Has anyone been in a similar situation? Was it a good idea to add a third person?