It would be very, very kind of you to let people know... especially since chicken pox is at it's most contagious PRIOR to the rash.
As simple as "We just found out we were exposed to Chickenpox over the weekend, and now my daughter has a fever, so we wanted to pass along a 'heads up'."
DO be prepared for both extremes. Those who get angry over something you had no control over / no intent AND those who want to have "chicken pox parties". Fortunately both extremes are the minority. Most will simply be appreciative at your thoughtfulness.
And yes. ABSOLUTELY limit exposure to people (playdates, park, nixed... try to go to the grocery store on without her, etc.) for a bare minimum the next week to two weeks.
Good job mama! There are soooooo many inconsiderate jerks out there who INTENTIONALLY take their sick kids out and expose everyone and sundry it's an absolute JOY to hear from someone who is so thoughtful!
[[ETA: I had chickenpox as a child (age 5) and it was MISERABLE. Most of my friends ended up with at least some scarring... but I lucked out with no scarring. HOWEVER, I'm adhd, and therefore have sensory stuff. I itched SO badly that I literally screamed myself voiceless for two weeks and was in tears almost constantly. I didn't sleep. I threw up from the smell of the stupid pink lotion. I can only imagine the hell it was on my mother. She finally resorted to sedating me. So while some would prefer to get it naturally, since it's rarely life threatening... I know I for one - and any parent of any child with sensory issues (adhd, SPD, autism)- is in LOVE with the vaccine. ]]