How to send invitation to Slack users:
Step 1: Log in to JIRA as an administrator
Step 2: Select your Current Project from "Projects" dropdown on the top menu
Step 3: Expand the left menu, click on 
Step 4: At the right hand side, scroll down until "Slack Integration" option
Step 5: Click on Users
Step 6:
a. Click on 
to all Slack Users
b. Click on Send invite for sending a direct message to a specific Slack user
Step 7: Write any message content or sending without inputting
