We are sorry, but the only way to get an account currently is by one invite from a riseup user. Spammers were abusing the previous system making it useless for our users are we were constantly blocked by other providers. Until we can find a way to defeat those spammers, you will need to find a riseup user and get an invite from them :(
Note, however, that an invite code is not required for the vast majority of services provided by riseup. The only affected services are email, chat, and one of our two VPN services (see https://riseup.net/en/vpn). The "Red" service (see https://riseup.net/en/vpn/vpn-red) is the VPN service that requires an invite code. The "Black" service (see https://riseup.net/en/vpn/vpn-black), which provides many other additional features including ease of use and greater security and is the focus of our development does not require an invite code.
If you cannot acquire an invite code and need one of the three services that require it, or if your interest is in a service like riseup rather than specifically what riseup itself has to offer, you may want to look at our list of similarly minded radical servers (https://riseup.net/en/security/resources/radical-servers).