Normally you would use a co-signer when the person qualifies but for some reason to make you nervous, like new in town, no rental history, etc. 

He wants my involvement as a co-borrower and I think this would be a great opportunity to get involved on a deal at a very low entry cost. 3.5% down while rates are at 2.7% doesn't seem like I could go wrong.  

However, most of them will not have enough life experience to have any significant credit history.You are on the right track, but I would suggest you use a guarantor rather than a co-signor.  

If the students have no credit then I require they have a co-signer (like I mentioned before I don't have the option of grad students with established credit).

Tenant had a lease allowing 2 adults and 3 kids.( though only he lived in the house) Unknown to me he rented out a room to a co-worker.

I talked to a co-worker and she told me I should try to have the city buy my unit from me.

If it's just an agreement between the two parties, no way - I'd make the payer be a co-signer without residence privileges instead. 

When sending an application to a prospective tenant, what is the difference between a Joint Applicant and a Co-Signer?

I got turned down by several landlords who wouldn't sign a lease without a co-signer until I found a great landlord who took the time to understand my situation.

Also this was a co-worker in the sense of 15 people in the office and we spent an hour or so talking each day in group/1 on 1 convos, NOT a co-worker in the sense of "oh you work for this company?