So I've decided to try this out but I am stuck...
I have a Sonoff Zigbee 3.0 Dongle Plus-P and a Rapberry Pi3 running HAOS. Ive installed the Mosquitto Add-on and Zigbee2MQTT Add-on. It seems that everything is functioning up to this point. Where I am stuck is.... How do I join this thing to my Hubitat Zigbee network now?
You can't. Zigbee2MQTT is another Zigbee coordinator (like Hubitat). The coordinator runs the network, and you can't join two together. The two can run separately at the same time, or you can pick one or the other as your Zigbee coordinator if you don't want two Zigbee networks. (I have multiple Zigbee networks in my home, mostly just my Hubitat hub and my Hue Bridge network. I once had a few things on Z2M as well, though not via HASS.)
If you do want two but still want the HAOS Zigbee2MQTT devices in Hubitat, there are community options, including this. Note that this is just essentially a way to send device information (states and commands) back and forth -- nothing to do with Zigbee per se.
Or, just to make sure: do you want to just use the Sonoff dongle as a repeater? If so, you'd flash it with "router" (repeater) firmware instead of "coordinator" (hub-ish) firmware, then you can join it to Hubitat like any device. The device can run only one or the other at a time. This would not involve Z2M at all (unless you have another coordinator there and want to join it as a router/repeater to that network instead).
Hey Robert, Thank you, I curiously clicked on the link, it lead to a discussion about semphore, and I think you meant the Z2M app to connect HE & Z2M, that I can't locate
Thanks, I've updated that now. 
(I think something is corrupt with my browser history since it sometimes mismatches titles and URLs, and I pulled that one from a correct title without actually loading the page to see...I have not looked if the other Firefox user has reported a similar problem or if some day I'll just try clearing it and starting over, but I digress.)
Still seems to be incorrect?
Should be actually fixed now. 
Dang it. I installed Home Assistant on this darn thing and I just can't find any use for it. I was hoping to figure out what all this MQTT stuff I read about is useful for but it mostly just seems like I have no real use for it anyway..
You can add some zigbee devices to your Z2M setup. But if they work on Hubitat directly, there is no point adding all this overhead for no gain.
Yeah all my current devices work just fine. I have a few chatty radar devices that I could test on it but I think Id rather not bother with 2 zigbee networks in a congested 2.4ghz environment. Guess Ill just skip Z2M for now as it seems unnecessary.
Maybe I can put home assistant to work with influxdb or Grafana and export my Hubitat logs or make pretty CPU graphs like all the cool kids do.
What was your original goal? Are there devices that are not compatible with Hubitat that you’re trying to join?
MQTT can be helpful in a lot of different ways all by itself. For Z2M, it’s a necessity, but most of us I believe run Z2M so we can add devices that are not supported by ZHA in either platform.
This is a good use case. I have very chatty mmW devices on HA and it doesn’t seem to be bothered by it, but I’ve heard from other people, that on Hubitat they seem to cause issues.
If I'm being honest, the only real goal was to figure out what MQTT does and why it's useful. I have Home Assistant on this Pi3 and I am mostly just determined to find some use for it. So far all Home Assistant has been useful for is that it automatically detected my printer and nicely displays the remaining ink levels.... beyond that I haven't been able to figure out what to do with it.
If it's not obvious, this is all mostly just a way to fight boredom. My Hubitat is working just fine now so I need something else to kill the idle time.
About two years ago I added HA to my setup because I wanted to use BLE Beacons.
Immediately after HA was running I realized, I can convert many cloud-based integrations
into 100% local and forget about these much heated clouds. Ecobee Thermostat and
Switchbot Curtains instantly became local integration. My latest additions are Sonoff NSPanels replacing ZEN32 Scene Controllers. Not only I can create very nice looking Butons
with Labels and Icons but also I can display many other useful info.
So, HA is a very BIG help in my current setup.
Did you get this working? And which beacons did you go with if you did?
Yes, in addition to the HA I had to use ESPesense (it was easy to flash). I forgot which Beacons
I was using but whichever was on a ESPresence compatibility list. Everything was working
very well but I stopped using BLE Beacons because they have zero security and my use
case was for automatic door unlocking and opening. Now I am using Active RFIDs instead.
Their list included Chipolo One, which did NOT work. All the others appear to be unknown brands from AliExpress and I am unsure which to go with. In my case, they are just for presence detection. I'm currently using a few SmartThings arrival sensors on Hubitat but they are too expensive and go through batteries too quick to hand out to people like my parents or housekeeper..
I used Blue Charm BLE Beacons:
https://www.amazon.com/gp/product/B085XN9B7N/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
They are nice looking and worked very well. But ALL BLE Beacons are not secure and that
was a reason I stopped using them.
Download the Hubitat app
