Whether your friends are seasoned Discordians, or have never touched the app, inviting them to your server is a piece of cake (and no, I'm not lying).
"RYUU GA WAGA TE-" sorry friend that won't work here.
Open your server and click the Instant Invite icon next to the channel name.
A box will appear with an invite code which you can send to all you friends. Simply copy, paste, and send to any and all of your unsuspecting subjects (er, friends) and begin operation world domination!
Note: Each channel in your server has it's own Instant Invite link, and which ever channel you pull the link from is where the user will land on your server each time they try to access it. For example, if you send Glados an invite link from your #rules channel, then every time she accesses your server the #rules chat channel will be the first channel she sees.
However, if you're frustrated and tired from figuring out which Portal to take, you can always send your friends an invite by right clicking on their icon from anywhere in Discord then selecting Invite to Server.
As long as you have the Create Instant Invite permission for your role on a server, you'll be able to see the server in the drop down menu that appears. Once you select the server, a DM will be sent to your friend with the invite!
Lastly, if your friend has never used Discord before then you can send them the invite link anywhere you can paste text. Upon clicking (or, copy/pasting) it, they'll be prompted with a quick rundown on Discord including an opportunity to set up an account.