I'd have to look up the official rulebook, but if I'm not mistaken, Player A should not only declare it's a group of eights, but also specify the exact colors (e.g. 'red 8' and 'green 8').
This is further supported by e.g. this page:
However, a player who has in hand the tile which is represented by a joker on the table, the player can substitute the real tile for the joker and then re-use the joker in some combination on the table.
(emphasis mine); there's only one tile it represents.
Since you've tagged it house-rules, there is room for some variation on the rules. Some of them are discussed in the linked page; when I was a kid, we played it in a more liberal way than you describe; during your turn, you could use the joker for whatever tile you want (i.e. you don't need to substitute it), as long as the final configuration consisted of valid sets only. That meant e.g. that if you had a blue run of 1, 2, 3, joker, 5, 6, 7, you could basically take out the joker and do whatever you want with it.