If you are designated as a collaboration administrator, you may enable disabled security zone members.
To enable a disabled security zone member, bring up the the Unity interface (see Displaying The Unity Interface). Then expand the "Collaborations" node, expand your collaboration node, expand the "Security Zones" node, expand your security zone node, right-click on the security zone member to bring up the security zone member context menu, and click "Enable".
You must be a collaboration administrator within the current collaboration in order to enable a disabled security zone member. If you are missing this option, ask your administrator to give you collaborator administrator privilege within the current collaboration.