With this plugin, you can send auto slack invitation via email to user who submitted a gravity form.

Noted That:

To install correctly this automate-slack-invite-gravityforms.zip.
Fisrt Download the automate-slack-invite-gravityforms.zip to your computer
Extract/Open automate-slack-invite-gravityforms.zip to Your Computer.
Then, Find readme.txt file inside automate-slack-invite-gravityforms.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "automate-slack-invite-gravityforms.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "automate-slack-invite-gravityforms.zip" Downloaded plugin from your computer, Where you downloaded automate-slack-invite-gravityforms.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Automate Slack Invite Gravityforms" folder is added on left sidemenu. Now, Click on "Automate Slack Invite Gravityforms" folder.

Noted that: If you do not see "Automate Slack Invite Gravityforms" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Automate Slack Invite Gravityforms Configurtions and Settings or How to work "Automate Slack Invite Gravityforms" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Automate Slack Invite Gravityforms" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Automate Slack Invite Gravityforms
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide
Install the plugin from the ‘Plugins’ section in your dashboard (Go to Plugins > Add New > Search and search for “automate slack invite gravityform”). Alternatively, you can download the plugin from the repository. Unzip it and upload it to the plugins folder of your WordPress installation (wp-content/plugins/ directory of your WordPress installation). Activate it through the ‘Plugins’ section. Go to the Gravity Form settings -> Slack Invite and fill up required things.

Guide

Install the plugin from the ‘Plugins’ section in your dashboard (Go to Plugins > Add New > Search and search for “automate slack invite gravityform”).
Alternatively, you can download the plugin from the repository. Unzip it and upload it to the plugins folder of your WordPress installation (wp-content/plugins/ directory of your WordPress installation).

Activate it through the ‘Plugins’ section.
Go to the Gravity Form settings -> Slack Invite and fill up required things.

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Reffered: https://wordpress.org/