I find it is better to open Discord from their web site rather that downloading it to my computer. There is also a mobile app that you can download to your device.
In order for someone to reach a server they need a Discord account. Once they have an account anyone on the server who has permissions can send them an invitation. At the top of the list of channels is a person icon with a plus sign (+). Click on that icon to get an instant invite to post for other to link to the server.