Achievements and quests will not be counted. It is really lame, because of the people leaves all the time in low prio, so there is no fun playing the game Community Help Post or search in Steam Discussions for an answer to your question.
Joining a group with another individual that is in the low priority matchmaking queue will cause the entire group to be placed in the low priority queue until the group is disbanded or the low priority individual leaves. Extended AFK behavior or otherwise not participating in a game that you have joined. Queuing with other accounts will place them in the Low Priority matchmaking pool while partied. Battle points or other seasonal rewards cannot be earned.