As Scott mentioned, depending on which state you are in and claims you go to, you may have to have a permit from USFS or state agency.
As to claims, just show up when convienient. You might run across other members, or not. If someone is there, ask if they are members of the Association. I don't say GPAA, the answer is always yes, half the time the answer to just Association is "what association" or "huh". Then I know they aren't members. You might be asked as well. Just show your card and you are good to go. If you ask someone and they won't show membership, remind them that it is an active mining claim, for members only, and politely ask them to leave. If possible get descriptions and vehicle info to pass on. But don't put yourself at risk, not worth it. If it's late in the day and they seem like nice folks, I'll let them stay and invite them to our chapter meetings. Got a couple of members that way.
If you run across members, they will be more than willing to help and show you the ropes of prospecting, and don't be afraid to ask. If you have a chapter near you, consider getting to know them.....more opportunity to learn and have fun in a family setting.