@Ilsa On some Mastodon servers (instances) there might be some limitations on registration.
That's not the case in other places, and here at Qoto.org, it's always open to new members. No invite needed, if she wants to join here, just open the webpage, and select register an account.
@Ilsa give them a pointer to https://qoto.org/about
...and all you need to do is register via this panel :