How about this.
1. On the satellite there's a 9600 FSK regenerating repeater. If it hears a bit, it sends a bit without any other smarts. 2m up, 440 down.
2. Also there's a clock. It puts out a tick 8 times per second.
3. For the ground stations, we design a simple TDMA system. Each user gets a time slot for transmitting into the satellite. Let me see, 9600 baud is 960 characters a second, so we break that into 8 blocks of 100 character or so every second and for up to 16 users for a 2 second time block. Enough bytes to do a small ax25 packet, or maybe we just stew up something new. Callsign + checksum + data.
3. Then we just make it a big chat room in the sky. The thing flies over, you type on your computer and then your software uses an unused time slot for sending the data. Everyone can read your message, and can chat back to you.