If you are designated as a collaboration administrator, you may mark deleted collaboration members as included.
To undelete a 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 "Mark As Included".
You must be a collaboration administrator within the current collaboration in order to mark a collaboration member as included. If you are missing this option, ask your administrator to give you collaboration administrator privilege within the current collaboration.