Hi there one post wonders. It is my opinion players can actually affect the aggro cap (just bring a pet, that's one way). Holding aggro, well that's a different story.
Troof, there is currently a very clever aggro cap which is quite high. It is (player + #teammates + #pets) * 17 = Aggro Cap "(the limit is unknown at this time)"
"The aggro cap for a single character is 17 enemies. A character can get the attention of others, but only by shedding the aggro of one of their current enemies, either by defeat or by extension: attacking a non-aggroed enemy will release one of the currently-aggroed enemies from the character's aggro list. A character with pets can aggro more than 17 enemies, since the pets have their own aggro list (the limit is unknown at this time)."
Sooooooo if a player or team has enough pets they could theoretically aggro way too many targets.
The way targets behave currently could easily overwhelm even the toughest characters.
Note: Good luck keeping those teammates alive.
ps: I'm surprised no one mentioned +#/8 as a way to increase spawn sizes. I guess that's not aggro or hit caps.