Logging Flight Time Question

Couple questions as it pertains to logging time.

1) What is a way that a person can log time (and what type of time)
sitting right seat in an aircraft certified for single pilot ops?

2) Can a person log the time spent right seat in a pressurized
aircraft without having a High Altitude Endoresement (assuming all
other currency, and endoresments requirements are met)? Or is that
endoresement only required if your going to log PIC time?

For the record, the aircraft in question here is a CitationJet.

I had another pilot friend ask me this the other day, and I've never
been good with endoresements, so I want to make sure I give him the
right answer.