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:
- Click on Manage Workspace cogwheel icon.
- Click on Members.
- Choose Invited Users, in the Show drop down.
- Tick the boxes of the people you want to re-invite.
- Click the Invite button.
You can't re-invite people within 1 hour of inviting them!