Think of a trunked radio system like a line or queue at walmart. Each frequency (repeater) is an open cash register. Everyone who wants to check-out gets into a line or queue. As the line moves along each person wishing to check-out selects one of the 4 or 5 registers to check out at. But in a trunking scenario each time someone comes through the line might end up at another register. So everyone needs to have all of the frequencies programmed in as a trunked system, because the control channel will tell USER A to go to FREQUENCY 3 and talk on GROUP FD. And when they unkey, the next user might be assigned to FREQUENCY 1 and talk on GROUP FD.
The one unique thing about Motorola Trunked systems is you can put your scanner in C-CH Only mode, and so all you need to program is just the red frequencies from the DB. The scanner will automatically select the in-use voice frequency. So if the trunked system you want to monitor has 2 red CC frequencies, program them in as MOT800 or P25 if it's a P25 system. And then under Group, program in the talkgroup ID number for the agency you want to monitor. Then set the scanner to SCAN mode instead of SEARCH mode. If you are in SCAN ID mode you'll only hear radio traffic from any talkgroup ID's you programmed in under Group. If you're in SEARCH ID mode you'll hear E-V-E-R-Y-T-H-I-N-G and that might drive you nuts if it's a busy system.
Bottom line, in the 325P2 program only the RED frequencies as a C-CH Only trunked system MOT 800 if it's analog, P25 if it's Digital. And then edit the GROUP and add all of the talkgroup ID's for whichever agency you want to listen to, and set the scanner to SCAN ID.