Almost job offer but what should I tell other companies I'm interviewing at?

I went through several interviews (phone + video), and I'm being asked to meet the hiring manager and his employees in person as more of an informal meeting to get a better idea. They mentioned basically they'd be giving me an offer but just that they'd be interested in seeing me. Once given an offer, they would like me to start fairly quickly (within a week or so likely)

I have several other companies I'm interviewing at where I'm near the end stages and am waiting for them to set up on-site interviews. Can I email them to let them know I have an offer (even though I don't quite yet) to speed up the process? What should I do?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official