The biggest reasons for people not being added are that they've submitted a consumer account and not an M365 account (Viva Engage requires us to use an AAD account) or that their host org has blocked Yammer. I've also seen issues where the invite e-Mail has been blocked too, so it's worth trying the link https://aka.ms/MacAdmins just in case, you might have been added but not received (or already have) an external account with us.
The community currently has just over 1100 people, we're adding them as quickly as we can. We're also working with our Viva Engage team to make this better as we go, but we're stuck with what we have for now.
There are no exclusions, nobody is being blocked or denied, we're trying to add every M365 account that hits the mailbox.
Thanks and apologies for the frustration here!