This is difficult because it depends more on them than you. they should not put you in an awkward situation by expecting any reaction from you other than silence ... but, "witnessing" and "proselytizing" sometimes borders on the obnoxious so the only advice is to be silent and/or excuse yourself from any debates.
nod your your head
or just say bless
or may we all be blessed
Just replace the word God with the word good. Good is the final wish of every goodwilling person, regardless their belivings about religion.
If you do not believe... and you are truly strong in what you believe...it shouldn't bother you. If someone says they do not believe...I'm doesn't matter to me, It does not make me uncomfortable. If I have to say my own prayer to myself...I will. No biggie.
One other way to look at it...Good friends do not push beliefs on and respect what you believe...however, good friends also want to tell you about God...it's like a double edged sword. Either way, I do not think they mean harm unless of course, they go above and beyond...in which I would say time to get some new friends :)
Bow your head and think about your grocery list. Thank you. I wish you the best as well.
Be yourself, If they pray, excuse yourself from it. let them know that you will respect and allow them to practice their faith in your presence but would like them to respect your beliefs as well.