You should just exchange your public keys with your friends. The software makes this easy: it can create an email with your public key and open your default MTA for sending it to your friends. Then they can easily import the received email (with your public key attached). Then, of course, they should send their public keys to you.
There is no problem being behind a NAT, it supports UPnP / NAT-PMP port forwarding.
Or just get used to using decentralized and encrypted p2p communication solutions. No server to set-up, just the client to install.