Two thoughts.
1. You could have the cost for the power be less than the benefit since it is restricted just for sponsorship. The catch being only one sponsee.
[-1] Minor Sponsor (You must take this power again for each person you sponsor)
Power Broker - You may make a pact with a person to lend power when it is needed. The strings attached to this pact are up to you.
Credit Limit - You may offer up to two fate points to another through a pact. You own the compel attached to each fate point given and must use it to the purpose of the agreed upon strings of the pact. A person may not owe you more compels than their credit limit. Once a person's credit limit is reached, a compel must be paid before more debt can be accrued.
Breaking Pacts - At any time, you may choose to break this pact. All remaining debt must be collected at that time. Once the debt is discharged, this power may be removed and the refresh spent to buy it refunded.
This could be tiered like the building block powers, but it's a little unwieldy that way.
[-2] Major Sponsor
Credit Limit - 4
[-4] Godly Sponsor
Credit limit - 8
[-6] Supreme Sponsor
Credit Limit -12
2. A less unwieldy option for multiple sponsees, you could have the cost be the same, but have an unlimited number of sponsees with the limit the same for each.
[-1] Minor Sponsor
Power Broker - You may make a pact with any number of people to lend power when it is needed. The strings attached to this pact are up to you. The credit limit for each person is the same.
Credit Limit - You may offer up to one fate points to another through a pact. You own the compel attached to each fate point given and must use it to the purpose of the agreed upon strings of the pact. A person may not owe you more compels than their credit limit. Once a person's credit limit is reached, a compel must be paid before more debt can be accrued.
[-2] Major Sponsor
Credit Limit - 2
[-4] Godly Sponsor
Credit limit - 4
[-6] Supreme Sponsor
Credit Limit - 6
EDIT: You might also give option 2 a surcharge like Modular Abilities if you feel the need.