SEI is a company based in Oaks PA. The manage around $130B (or around there) SEI made their name in the trust business in that they produce the software that the vast majority of banks use for their own internal accounting.
Through aquisitions they got into the money management business at first on the pension side. They manage the DB & DC pensions for many fortune 500 companies. They also developed their own model for managing money. Other than laddered FI they don't run any of their own money. They have a team that evaluates third party money managers and then SEI turns that into a wrap for individuals. They also run a managed account platform with the same managers for larger accounts.
They do not take money directly. The only way to access them is through a registered investment adviser. Te expenses can be a little higher because you have the manager fee + SEI's fee + the advisor fee. They do have a central focus on asset allocation, then on manager selection.
On top of all of this, better than investing with them was investing IN them. for the 1980's 1990's and early 2000's the stock (Symbol SEIC) was one of the best performing public companies on many lists.
This story is a little old, but it tells you about the company http://www.fastcompany.com/online/14/totalteamwork.html
Almost forgot... SEI also does a lot of back office reporting and settlement for mutual funds. I think they do it for almost 2 TRILLION dollars of mutual fund and ETF money. They do the back office, settlement, and reporting for all of the Ishares ETFs