In ultras, a crew provides support. They follow you around the course, usually in a vehicle. They meet you at predetermined spots, often aid stations. They may provide support in the form of supplies, tracking your pace to keep you ahead of cutoff times, helping you solve problems that may arise, and providing general moral support. They will have the ability to make sound decisions in the later stages of a race, well after you've lost the ability to think rationally.
So is a crew necessary? No. Are they extremely helpful? Yes. I've only used a crew in my 100 mile races. In my 50 milers, I was able to complete them without outside help. Generally speaking, the longer the distance, the greater the need for a crew.
In my first 100 mile attempt, we were all rookies. My two crew members had no significant running experience, let alone ultra experience. We did a lot of learning on the fly. Despite their inexperience, my crew did a great job.
In my second 100 mile attempt, I was better prepared. One of my crew members from the previous attempt crewed again, so I had some experience. The rest of my crew were either experienced runners or had done lots of research on the art and science of crewing. There was still a learning curve for the first few aid stations, but my crew did a wonderful job. I would not have finished this race without them.
Okay, you decide you need a crew. How do you find crew members? I lured my crew with promises of a fun adventure where they would have ample beer... essentially it would be a 30 hour party. I failed to mention the lack of sleep, the rushing from aid station to aid station, and dealing with me in my absolute worst state.
Friends and family are always a great option. If they have running experience, that helps. If they have ultra experience, that's ideal. If they are also willing to pace you, do anything to get them on your crew.
If you are having problems finding crew members, sometimes online running communities can be a good source of potential crew members. Less experienced runners may be looking for an opportunity to learn. More experienced runners may be looking for an excuse to get out of the house. Others may be familiar with the course.
Once you have your crew formed, it may be useful to designate someone as the "crew chief." This will be the person that makes all the final decisions. If you have a crew of several people, organization and coordination may wane in the later stages of a race as fatigue sets in. Having one person ultimately responsible for decisions can help keep the crew operating as a singular unit.
This July, I'm planning on running the Burning River 100. This is the race I DNFed in 2008. I'll be recruiting a crew once again. I haven't explicitly asked them yet, but I'm hoping to assemble as much of my previous crew as possible. I may have to sweeten the pot this time. I may have to bust out promises of a traveling keg and a visit to the local gentleman's club.