Can a sole prop include husband and wife as joint owners? Does the spouse have to be just a "signer" or can they be joint owners?

They can in Washington State (only if they're marries though). Don't know about other states though.

Not in Georgia

They may be able to register the assumed name as a sole prop in the State in which they are conducting business, but from the IRS standpoint, they are either a partnership or a qualified joint venture.

Most states would say no even for community property states, as the official ownership is a sigle person, with rights of the community property, but that is a state issue. IRS is different as noted by rlcarey.