Part 65.61.d.2 IFR Requirements
If you're under the hood in VMC + VFR, you could have an appropriately rated pilot (i.e. PPL) in the other seat as the safety pilot. If the other pilot is instrument rated (but not a CFII) + is the PIC for the flight, you could even be the sole manipulator of the controls in IMC + IFR, although that gets into the questions about logging PIC vs acting as PIC, which is its own set of issues. The "safety pilot" option is probably more typical.