So what does this team need to do, which is not covered by db_securityadmin and db_accessadmin? Without this information, your question is somewhat difficult to answer.

I note in Books Online that it says: "Adding members to fixed database roles requires membership in the db_owner fixed database role." And that's a good thing, or else the team members could add themselves to db_owner.

Maybe the best if you give the team a copy of the database, and then you can use a tool like Red Gate's SQLCompare, or SQL Server Data Tools to replicate permissions and other security-related things from their copy once they are done. Or simply ask the team to produce a script that you can review.