Allow registrations only by invitations

Yes. You should be able to do the following:

1. Disable user registration. You should be able to do this by ensuring that BP’s Register component doesn’t have an associated page. (Make sure you do have an Accept page.)
2. In Dashboard > Settings > Invite Anyone, ensure that “Allow email invitations to be accepted even when site registration is disabled” is checked.

I have the same issue i i allow email invitations to be accepted even when site registration is disabled but still i m not receiving any mail i try to send to user gmail account other account too but none of them is receiving the link or any mail