added to a group, but don't have root

[mixmix]

ssb-tribes listens for group/add-member messages and if it sees on the first thing it does is look up the root message for that group. It needs to do this to derive the groupId (currently) so that it can register the groupId and key in the keystore.

if you don't have the root message yet, then all this breaks and the app throws at the moment.
If we don't throw then the user will never register the group and so will never rebuild / be able to decrypt group messages, be able to encrypt messages to the group

[mixmix]

Possible solutions:

1. defer processing - would require storing the unprocessed invitation
2. include the groupId in the invitation - this isn't ideal, as you'd have to trust the inviter rather than deriving the groupId yourself
3. include the rootAuthor - this would at least let you know if you are out of range of a group