Hi Felicita,

Invite codes are currently required because the exchange is still in private beta, that restriction will be removed soon.

As for Nimiq, a lot of people still don't know about the project yet, because the team have been flying under the radar with no marketing, they mostly want people to discover the project on their own. I'll say it's one of the top tier projects people should pay attention to before it's too late. They have a stellar team with their own dedicated blockchain.

Cheers!