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