What actions can a player take during their teammate's turn in 2v2 games?

Ruling: On your teammate's turn, you can cast spells and activate abilities (including legend abilities and gear abilities). You cannot play units or gear, perform standard moves with your units, or move your units to battlefields controlled by your teammate.

Sequence:

• You may cast any spell
• You may activate any ability (including legend abilities like MF and gear abilities like Garbage Grabber)
• You cannot play units or gear
• You cannot perform standard moves with your units
• Your teammate also cannot perform standard moves with your units

Nuances:

• Your units cannot move to a battlefield that has units controlled by your teammate (interaction with Charm effects unclear - may not be allowed to target or may fail)
• When a card says "you" it refers only to the card's controller, not your teammate
• There may have been a rules update that removed the activated abilities permission (noted by godhenry007 but not confirmed)