How invite links work, and the difference between a regular member and a sub.
There is no email invite system for regular members. The owner shares a link, and anyone who opens it joins.
Every band has two separate invite links, both on the Members page. One adds people as regular members, the other adds them as subs. They can be regenerated independently, which immediately invalidates the old link.
Anyone holding the link can join, so treat it like a password. If it leaks, regenerate it from the Members page.