To invite others to join your company, simply click the 'Invite User' button at the top right of the Members table on the Company Management page
After you click it, you will be prompted with the Invite User Dialog box
Enter the email address of the person that you want to invite and click the 'Create Invite' button. Once that goes through, you will be given a link that you can email to the person you want to invite. To the right of the invite link field is a copy button. Clicking this will copy the invite text to your clipboard.
If the user you are inviting is already a Graffle user, they will see your invite in the My Invites section in their User Profile page. More on that later.
Once they Accept your invitation, they will show up in the Members table of the Company Management page.
You can see a list of pending invitations on the Pending Invites table in the Company Management page. If the user has not accepted or rejected an invite they will show up here. If you would like, you can also cancel the invitation and they will not be able to accept it using the link you emailed them nor will the invite show up on the My Invites section any more after you cancel it.