Do NOT join SAG just for the sake of doing extra work.
Do it when you are ready as an ACTOR to take that next step.
It's a question of priorities. Do extra work if you like of- course- but it can't play a part in defining your goals as an actor if that is what you want to really do.
Are you an actor or an extra? Either is fine if it makes one happy but that's what it boils down to.
For example, if one wishes to make a steady living indefinitely as an extra, it makes sense to join as the jobs pay more.
HOWEVER, if you ultimately wish to ACT, it can be a real hinderance to join too soon as you need to be taking the opportunity to do as much non-union work as possible to get experience, credits, build a reel and network. The moment you join, you will find that this all becomes a lot harder to do. When you are competing for PAID union gigs, the competition is fat greater.
Pro bono stuff is necessary for everyone in the beginning but you can not do any non-union work once you join and that can be very detrimental for a new actor's beginning years.
""I'm not afraid to die on a treadmill. I will not be outworked. You may be more talented than me. You might be smarter than me. And you may be better looking than me. But if we get on a treadmill together you are going to get off first or I'm going to die. It's really that simple. I'm not going to be outworked." -Will Smith