Log in to the Kinetic Panel and head to the overview page.
You will see a Connection Address, which is your server's IP Address.
From the main menu, click on the "Multiplayer" option and select the server browser
In the Multiplayer menu, look for the "Direct Connect" button and click it.
Paste or type the server's IP address into the input field. Aswell as the port from the overview page
Click the "Join" button to join the server.
Arma 3 guide
How to join your Arma 3 Server
Joining your ARMA 3 server will allow you to play with friends as well as host more intensive missions