It's been a while since I was anything below but host, so please forgive any errors that I might make in regard to permissions.
An alliance can have:
1 Host
2 Vice-Hosts
5 Presbyters
10 Officers

Host has all permissions and is the titled leader of the alliance.
Vice-hosts have most permissions except expelling or demoting the host.
Presbyters have permissions to invite and accept new members and I am not sure what else (sorry)
Officers can invite new members.

I am not sure about diplomacy options as I never got involved in that area until I became host and had responsibility for the entire alliance. I think that vice-hosts have most or all diplomatic options available as well.

There is no game benefit to any of these positions in regard to prestige or honor and in fact it can be detrimental to your own advancement if you have to respond to the needs of your members instead of carrying out something for your self.
Having said that, it is an enhancement to gameplay for those who enjoy being more interactive and like to take some responsibility in a group.