DGA VS PGA
There are two guilds you should be aware of in this position. The Directors Guild of America (DGA) and the Producers Guild of America (PGA). Each have membership which do the lob of a Line Producer and/or Unit Production Manager, although there are distinct differences.
THE DIRECTORS GUILD
The Director's Guild of America (DGA) covers much more than only the Director. The DGA's membership is also comprised of Assistant Directors, Stage Managers and Unit Production Managers. Indeed, there are various stipulations where Location Managers and Production Assistants can join with those titles, but this is not the forum to discuss those nuances. Here, we focus on the UPM.
Joining the DGA can be a complex, harrowing and lengthy ordeal. Unless you are lucky enough to be Grandfathered in, you'll need to "count days". Grandfathering is when you are working on a project, and it "turns" union. You were already part of the project and the DGA will include you into it's fold. However, this is not always a golden ticket, and there are stipulations surrounding this too. For more information on grandfathering, please contact your local DGA office - then wish on a star.
I will quickly mention that another way of getting into the DGA is to go the route of "DGA Trainee". This is an 'elite' program where individuals learn the fine art of being an assistant director, then after a few years - they are in. The catch here is that you would join as an assistant director - not a UPM.
For the more practical minded, "counting days" is what you'll most likely end up doing. There is no shame in this (or at least I'll keep telling myself that, because it's what I did). As an overview, there is what's called a "qualifications List" and to get onto this list you need to perform the functions of a UPM (with all the proper documentation/proof) for 400 days. These days have a split of 25%/75% between Prep & Wrap / Shooting. Which means, 300 days are on the set shooting days, and the rest are counted for pre- and post-production. You need to gather these days on NON-DGA projects. For example, you work an a project for 6 weeks of prep, 32 shoot days and a week of wrap - you can reduce prep and wrap days by 35 and shoot by 32, which leaves you with 65/268 days to go. Good luck, you're almost there. Don't fear - many people have done it. It took me almost six years - but here I am.
Well, not quite. Here's another technicality. Once you gain a 25%/75% split totaling 120 days, you are able to join as a "Third Area QL". This means you can work anywhere in the world, except Southern California and New York (here's a map). However, you are still a member of the guild no matter where you are listed.
All of the rules and oversight to counting days is managed by the DGACA, DGA Contracts Administration. Not to be confused, the DGACA is not actually part of the DGA, they work semi-autonomously.
THE PRODUCERS GUILD
The Producers Guild of America (PGA) is the other group which covers the entire "producing team". This definition inherently encompasses everyone from the Executive Producer down to the Production Coordinator, which sticks the UPM and Line Producer somewhere in the middle. It's an important note to make in the beginning that the PGA does not officially recognize the Line Producer position, and while membership is allowed based on UPM work, the PGA does not actually represent UPM's, other than to acknowledge they are an integral part of the producing team.
Why? Because the PGA is not part of the Collective Bargaining Agreement, they have no real jurisdiction over the DGA UPM.
Regardless, the PGA is a fantastic group and resource to network and share information. They, like the DGA, offer many educational opportunities for the membership. Joining the PGA is a little easier, however - you still need verifiable credits.