I have a General class license. If I'm working with an Extra class license holder, what do we have to do to allow me to work bands outside General, but inside Extra?
The operating frequencies of a station under FCC jurisdiction are determined by the licensing level of the control operator of the station.
97.105(b) A station may only be operated in the manner and to the extent permitted by the privileges authorized for the class of operator license held by the control operator.
For you as a general class licensee to have the frequency privileges of an extra class licensee, the extra class control operator must grant you authority to use the station but the extra class licensee must remain the control operator during that time. The extra class control operator is responsible for the proper operation of the station.
The station identification consists of the call sign of the station. In the case of a general using an extra class station, this requires that the call sign be that of the extra class station. You are permitted to add your call to the identification, provided it does not conflict with, or cause confusion regarding, the actual station identification.
97.119(c) One or more indicators may be included with the call sign. Each indicator must be separated from the call sign by the slant mark (/) or by any suitable word that denotes the slant mark. If an indicator is self-assigned, it must be included before, after, or both before and after, the call sign. No self assigned indicator may conflict with any other indicator specified by the FCC Rules or with any prefix assigned to another country.
You don't exceed your own license limitations no matter what equipment is available.
If there is an Extra-class licensee there then they can operate wherever they are authorized, also.
As to your question, they can delegate you to do the contacts as long as they are present and you ID as them.
To use their Extra bands they must maintain positive control of the station.
Relative to the above answer.
Quite often during "FIELD DAY" operation an Extra Class operator will "loan" his call to a club or group in order to let them operate on all available frequencies, no matter what class of license the individual operator holds.
Generally speaking, a "CLUB" will elect an extra class operator as "trustee" for such activities.
If the Extra is sitting where the controls can be readily accessed, they can still be considered the "control operator" and you a "participant". The "control operator" remains responsible for the transmissions in this scenario.
Technically to use an extra band, you have to use an extra call sign. So let's say your call sign was K1aaa, and your friends K2AA. You would have to identify by "K1AAA/K2AA", or something like that. Or just have your friend ID every 10 minutes.