To borrow someone else's computer to go online.
Or: The phenomenon of being mistaken for someone else simply because you happen to have Instant Messenger while using her computer.
I sent a really erotic instant message to my boyfriend, only to find out his 13-year old brother was guesting.

(guests)

n. 1) in general, a person paying to stay in a hotel, motel or inn for a short time. 2) a person staying at another's residence without charge, called a "social guest." An important distinction is that a non-paying guest is not owed the duty of being provided a safe boarding space, as is a paying customer. Thus if a social guest trips on a slippery rug, he/she has no right to sue for negligence, but a paying guest might. 3) an "automobile" guest is one who is a passenger without paying, as distinguished from a taxi fare, bus rider or one who has paid a friend to drive. However, the so-called "guest statute" may give a non-paying passenger the right to sue. An automobile guest is somewhat (but not entirely) analogous to the "social guest" in a residence. See also: guest statute invitee

n.

In show business, a guest appearance is the participation of an outsider performer (such as a musician or actor) in an event such as a music record or concert, show, etc., when the performer does not belong to the regular band, cast, or other performing group.

Oscar Montague Guest (24 August 1888 – 8 May 1958) was a politician in the United Kingdom, initially with the Liberal Party and later as a Conservative. He was twice elected as a Member of Parliament (MP).

A guest host (or guest presenter in the United Kingdom) is a host, usually of a talk show, that hosts the program in lieu of the regular host if they fall ill, have another project or commitment, or are unable to host for some other reason.