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 that Riseup support will not give out invite codes.
Note, however, that an invite code is not required for the vast majority of services provided by riseup including mailing lists (https://lists.riseup.net), VPN (https://riseup.net/vpn) and crabgrass (https://we.riseup.net/). The only affected services by the invite requirement are email and chat.
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).