Type the command /open once you've been transferred to your server. Then, you'll see some additional instructions, telling you to have your friend join the hub server at play.learntomod.com. The text will say: "You have opened your server. Tell your friends to join play.learntomod.com and run the command /join ##" The ## is a number that will automatically be assigned to you.
Then type in the /join command you see.
Keep in mind that 4 is the recommended amount of people per private server, and 10 is the hard limit. The servers aren't designed for more than 4, so anything above that will probably have side effects of lag.
If this doesn't work, redo the Minecraft Connect badge, making sure that you selected Multiplayer mode.