I started to use Join yesterday and I like what I see.
More information: http://joaoapps.com/join/
To make Domoticz notifications work with Join:
Go to:
Setup -> Settings -> Notifications in Domoticz web gui.
Look for:
Custom HTTP/Action
This is to send the notification to ALL or specific groups of devices. Look further down this page for sending only to specific device(s).
Enter the following:
#FIELD1: Enter your private API-key here.
Get yours from https://joinjoaomgcd.appspot.com/:
Click "Join API"-button.
Click the big orange button "SHOW" at "API Key:".
Please note the red disclaimer on the page:
"VERY IMPORTANT: this API Key gives everyone that knows it access to all your devices so be very cautious who you share it with. You can always delete or reset it so it stops working if it's ever compromised."
Copy the api key to the field named #FIELD1.
#FIELD2: Enter the URL that should open if you click the notification.
This is optional. I use the URL that points to my Domoticz web interface.
#FIELD3: https://googledrive.com/host/0B13H1MftDH4lbmdPeDBHV05UaGc/domoticz_logo_color.png
This is optional: This will set the icon used for the notification. This will also set the notification light color based on the icon. If you leave this blank, the standard icon for Join will be used.
#FIELD4: https://googledrive.com/host/0B13H1MftDH4lbmdPeDBHV05UaGc/domoticz_logo_white_trasparent.png
This is optional: This will set the notification icon shown in the notification area. To see the D in the logo, the D has to be transparent. This logo is also all white.
#TO: group.all
This will send the notification to all of your connected devices.
Device groups available:
group.all
group.android
group.chrome
group.windows10
group.phone
group.tablet
group.pc
URL/Action: https://joinjoaomgcd.appspot.com/_ah/api/messaging/v1/sendPush?apikey=#FIELD1&title=#SUBJECT&text=#MESSAGE&icon=#FIELD3&smallicon=#FIELD4&url=#FIELD2&deviceId=#TO
Do not change anything in this url. Copy as written.
POST Data: Not used. Leave blank.
POST Content-Type: Not used. Leave blank.
Make sure that you select the checkbox to enable the notification.
Press "Test". Did it work? If yes, don't forget to save by clicking the "Apply Settings"-button.
If it didn't work, start over.

This is to send the notification to one or more specific devices:
Enter the following:
#FIELD1: Not used. Leave blank.
#FIELD2: Enter the URL that should open if you click the notification.
This is optional. I use the URL that points to my Domoticz web interface.
#FIELD3: https://googledrive.com/host/0B13H1MftDH4lbmdPeDBHV05UaGc/domoticz_logo_color.png
This is optional: This will set the icon used for the notification. This will also set the notification light color based on the icon. If you leave this blank, the standard icon for Join will be used.
#FIELD4: https://googledrive.com/host/0B13H1MftDH4lbmdPeDBHV05UaGc/domoticz_logo_white_trasparent.png
This is optional: This will set the notification icon shown in the notification area. To see the D in the logo, the D has to be transparent. This logo is also all white.
#TO: Enter your specific device id.
Get the specific device id from https://joinjoaomgcd.appspot.com/:
Select the device.
Click the "Join API"-button.
Copy the device id to the field named #TO.
If you want to send the notification to multiple devices at the same time, enter multiple device ids separated by comma (,) in the field named #TO.
URL/Action: https://joinjoaomgcd.appspot.com/_ah/api/messaging/v1/sendPush?deviceId=#TO&title=#SUBJECT&text=#MESSAGE&icon=#FIELD3&smallicon=#FIELD4&url=#FIELD2
Do not change anything in this url. Copy as written.
POST Data: Not used. Leave blank.
POST Content-Type: Not used. Leave blank.
Press "Test". Did it work? If yes, don't forget to save by clicking the "Apply Settings"-button.
If it didn't work, start over.

Note:
It will probably work to just change the #TO-field from "group.all" to the device id(s) if following the first instructions but it's a bit safer to not use the api key at all.
Example use in Blockly: Resulting notifikation: