I created a group and added users for all people on my trip. Afterwards, I sent an invite out to members of the group. Upon joining, they selected their name. Then they would need to create an account. However, this would then add a new user to the group that had the same name. The fix for this was to kick the newly created user from the group. Then, once signed in, they could click the invitation link once again and click the original user and it would work.