If you look at the Members page (in the workspace menu dropdown) you can see who has an outstanding invite. To re-invite users, you can enter their email address on the normal Invite page, see Getting People Involved.

Workspace managers can do this more efficiently:

1. Click on Manage Workspace cogwheel icon.
2. Click on Members.
3. Choose Invited Users, in the Show drop down.
4. Tick the boxes of the people you want to re-invite.
5. Click the Invite button.

You can't re-invite people within 1 hour of inviting them!