Do level 8 players still gain AP? The way I understand it, level 8 is the highest level, so once you're there it seems like you gaining AP it doesn't do any good, and is really taking potential AP away from your local team mates.
Gaining AP doesn't benefit someone who is already L8, but they do continue to gain AP - although it won't be reflected in the octagonal progress bar, since there is no progress towards L9.
I have heard of L8 players who had enough AP (2.4mil+) to become L9 if that existed :)
The factor of taking potential AP away from team mates is definitely a valid concern though, as an L8 its better to just help in bombing runs and let lower levels finish off portals & fields for AP whenever possible.
Yes, you still gain AP when you reach level 8.
When new levels are added (and the developers have hinted that they will) the AP you already gained will count towards those levels. If you already have enough AP (probably 2.4 million) then when level 9 is added you should automatically become level 9.