TF2 Server Issues

I would like to create a TF2 server that me and a couple of friends of mine can join. Unfortunately, every time I send them an Invite to Join, they say I'm not currently playing on a server / server can't be found. My friends tried to create servers on their sides but had the same / similar issues when we tried to connect. What must I do or where do I go in order to make a server where my friends can play with me?