Over the last couple of days the mail system and open world group invites take several minutes to register.
It’s frustrating standing at a mailbox waiting for the mail to send off, but the invite lag problem makes trying grouping up with other players on-the-fly pointless. By the time your invite registers with the other player, the task you are both trying to complete is nearly over or they have moved on to other things so you don’t compete.
Realise Blizz are likely aware of this but I hope it’s resolved soon.