If your device is eligible (I'm assuming it is if you've had the banner and been able to register) then Samsung release new members / updates in batches.
Some members advised that were accepted and got the beta updates in minutes, others took much longer.
I myself had to wait 8 days and I joined around beta 3 update time.
There is nothing you can do but wait. Rebooting your device, deleting and reinstalling apps, leaving beta then trying to rejoin will not speed it up.
S24 Ultra