When I cast Stealth or Invisibility on a teammate, do I need to keep him in sight (following him everywhere) to maintain the spell, or is LOS required only for the initial casting?
From Shadowrun FAQ:
Do you need to maintain line of sight (or touch, with Touch range spells) to sustain a spell? What about Permanent spells?
A spell requires a magical link (touch, sight, material link, symbolic link, etc.) to cast. There is, however, no requirement for the magician to maintain touch or line-of-sight while maintaining the spell, as the link between caster and spell is already in effect. By the same principle, you do not need to maintain touch or line of sight when casting a Permanent spell; the spell must simply be maintained until the spell’s effects become permanent.
One of the exceptions is when moving around an area-effect spell, cf Core rules:
If an area-effect spell is sustained, the affected area may be moved with a Complex action, as long as it remains within line of sight.