Unable to register new users / unable to invite contributors

Hello together, thank you for hosting the service.

I'm working on a project that's hosted on this site. The ppl who originally set up the team are no longer on the project. I'd like to invite the new contributors but they can not register accounts.

They are getting the error:

Email is not allowed for sign-up. Please use your regular email address. Check with your administrator.

I can also not invite them because then I'm getting this error:

The member's email address is not allowed for this group. Check with your administrator.

Is it possible to register at the moment? How do I invite them?