It varies although mostly, they all do marketing themselves rather than hiring in companies they recruit there own employees because marketing is a key part of the business and very important that they get it right.

Yes, they hire marketing companies to promote the film and to schedule interviews with the performers and directors. Here goes a list.
Ant Farm Los Angeles, CA (323) 850-0700
Artphase, LLC Los Angeles, CA (310) 943-9208
BLT & Associates Los Angeles, CA (323) 860-4000
Cimarron Group Hollywood, CA (323) 337-0300
CMP Burbank Burbank, CA (818) 729-0800
Crew Creative Los Angeles, CA (323) 468-3636
Firestone Productions Glendale, CA (323) 461-7500
Framework Studio, LLC Culver City, CA (310) 815-1245
GSM Entertainment Los Angeles, CA (310) 205-0370
Haley Miranda Group Culver City, CA (310) 842-7369
Hammer Creative Advertising Hollywood, CA (323) 463-9156
Indica Entertainment Advertising Los Angeles, CA (323) 850-1620
Insight EPK Agoura Hills, CA (818) 665-5451
KO Creative Beverly Hills, CA (310) 288-3820
Mark Woollen & Associates Santa Monica, CA (310) 399-2690
mOcean Los Angeles, CA (310) 481-0808
New Wave Entertainment Burbank, CA (818) 295-5000
Playground Media Group Santa Monica, CA (310) 315-3800
Studio 27 Hollywood, CA (818) 216-9026
Trailer Park Los Angeles, CA (323) 461-4232
Winston Davis Associates Los Angeles, CA (323) 930-8535