I believe the time is a total of 20 hours played to unlock chat, forums, fleets... Additionally, I believe it can be gone around by spending or having a balance of C-points (not sure of amount, probably small like $10 ).
According to the F2P Matrix, they should "Join a Fleet (Guild) Unlimited Access".
But if a Silver has the 20hour/C-Points restriction if they try to create a fleet.
I think what the issue could be that the Silvers can not "help you" create the fleet to begin with. If you formed a fleet with Gold members first, then the Silvers should be able to be invited afterwards.