I am having this issue, also.
We are unable to select join for either of us. Have tried logging in logging out, zoning in at the same time in and out of party.
The best case we get is if one of us is able to “Join a friend” when we select it it resolves as normal but we are unable to see each other. We just appear on each others mini map, the party icon is not updated and we can not see each other anywhere in the gamespace.