Had something like this happen once in City of Ash.

My three group members were waiting for me and I was following closely...

I, however, couldn't do anything...I could see them, they couldn't see me. Chat worked fine.

I could not aggro or interact, and via voicechat, I found out that on their map, I was still up on a perch some distance away.

I returned to the spot the game thought I was still at and all reset...

Only happened the one time.

Find the common factor (the odd man out) and have them retrace their steps to the location where the rest of the group sees that member. It may snap out of it. If not, /reloadui