Son of Turnip

Hi all, I never really noticed the impact of grey teammates until one disappeared before my eyes and I couldn't teleport them to me. What does the greyed out teammate mean, and how can I / my teammate get around not being able to teleport someone who should be standing 20 ft. from me? Full disclosure: this happens both on a multiboxed account (650 characters online, following the rules) and when playing with a friend. Thanks!