How to send invitation to Slack users:

Step 1: Log in to Jira Server 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 User

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