However they want a focus puller, PSM, and boom op, but want them to work for free.
There are particular jobs that just aren't fun on a set. Those positions can be rather difficult to get people to commit and stand by those commitments without a good reason. I've found 1st/2nd Assistant Camera Ops, Boom Ops, runners, PA's, even make up artists can be tough to get them keeping coming back on an unpaid gig. It does depend on the person. Around here it's rather easy to get a free Director/DOP/Lead Cast.
I don't know how to advertise for people to work for free and make it look good. Especially since my friends say that if they do not get those crew members in time, that they are still going to go ahead in shoot anyway. The movie is already cast, but they only have a DP/camera operator. So any ideas on how to get a crew members to work for free.
It can also depend on how high you want to keep the standards of the people you want to bring in. Are you looking for keen people with relatively limited (or none) experience or do you only want highly experienced people? If you're looking for the former, tell them that. Let them know you offer an environment of learning and experimentation.
If you're looking for the experienced, you need sell them on the production. It changes from "Show my why I should let you work for this production and where are your references" to "Here's 10 reasons you'll be disappointed if you miss out on this opportunity" Why should they be excited to join your production. Why will their life improve by donating their time to your cause. What will drive their passion. What will make them say yes, I'm in! If you have a name(s) on the project that may draw people, promote that fact. If those involved in the project are award winners, promote this fact. Some people will be drawn to projects like that, others won't get involved with free work, regardless. If your group is Farmer Joe and his redneck mates, expecting volunteer Academy Award winners to work on your "Aunt Country Hick video guide to roasting road kill", you may get a little disappointed.
Also, try starnow.com. It can give you mixed results, but it can work wonders sometimes.