If one of your users lost their invitation email, or can't remember how to log in, you can help them out!
Click the "Set up people" button at the top of your project:
You'll see everyone in the project. Click the ••• next to a person you want to resend an invitation to, and choose "Send a link to log in":
Voila! We'll send an email to your user who hasn't accepted their invitation. If they have accepted the invitation already, they'll get linked straight to the login screen.
If someone has been invited to your account, but they haven't joined yet, the admins and the owners of the account will see it in Adminland > Add/remove people or change their access, and be able to resend the invitation the same way:
NOTE: While an invitation to Basecamp is pending, the only people who can edit someone's info are admins or the person who originally invited them. Once someone has joined Basecamp, you won't be able to update their name or email address anymore. That person will have to make those changes themselves.