So, after much handwringing, I decided to give FH a go. 
 
Burnout Paradise was my last  flame (yuk yuk) in terms of driving games, so its been a while.
 
So far, loving it. However, between wanting to give single player a good go, and not having a large amount of free time, I'm worried by the time Im ready for multiplayer, everyone will have either:

1. Moved on to other games; or
   
2. The skill window will have moved too far along (i.e. the gap between someone just starting MP and everyone else will be too large to bridge).

(I know its only been a week!) 

So,  I thought I'd turn to the GB community playing FH... whats the MP curve like so far? Should I jump in now, or am I safe to leave a week (or three)?

I think you'll be fine to wait. The Forza games all have pretty dedicated communities, and Horizon has a pretty happy-go-lucky crowd playing it at the moment. Plus, the game doesn't put a lot of weight on being especially skilled at it. You'll have fun whenever you decide to jump in.

what he said and eventually you will want more $$ which you will need to play online to et

Forza Horizon does not have a big skill window as Forza 4 has. Besides that, there is always someone slower than you. ;)

There is always the option of joining the GB car club too, I think they're doing FH on Tues nights and FM4 on Thursdays.