Some of the members of your hub might not have signed on in a long time. Others were imported from other sites and have never logged in. As a hub admin there are things you can do to help your hub members get into your hub.
From the "Manage Hub" screen when you click a member's name you'll see their profile and a section called "Admin Password Reset"
If a member has been assigned a temporary password it will be visible here and you can communicate that password to him or her. If a member has set his/her own password you won't see their password (it is encrypted) but you can generate a new temporary password for them. The new password will be displayed and you can share it with them and even test it out and prove that it works.