This feature request has come up before and is useful for preserving COINSTAC privacy ideology. Currently a consortia owner can remove a member at anytime, but that still allows any member to join the consortia without any approval. Although the following workflow still allows non-members to see all the info relating to pipeline via 'pipelines' tab and the consortia members list via the other consortia tab, its just a better workflow in general.
Workflow:
Owner creates public consortia, invites members, then they join
non-members can send a request to join the consortia, but it sends an 'approve or deny' message to the consortia owner
If owner approves the member is added to the consortia if not they are denied the request.
Feel free to add comments for any improvements in the workflow.