Hi @matt o thanks for posting in the community! If you or one of your team members upgraden to a Team subscription you will be able to invite the other members to join the team by following the steps outlined in this help center article. 

For more information on the features available to each of our subscription levels please review our Lucidchart Plans article. If you are interested in upgrading right away please visit our pricing page.

I hope this helps! Please feel free to comment in this thread or submit a support ticket via this form if you have any questions - we're happy to help!