The work being accomplished at a meeting usually implies who should be on the guest list. But sometimes a little subtlety is called for.
Often it is easy to figure out who should be on the guest list for a meeting. For example, a regular department meeting would typically include everyone in the department; a regular project team meeting would typically include everyone on the project team, or the subgroup responsible for the work being discussed.
Some meetings are intended to result in decisions, while others are intended to generate ideas. Employees who are more junior often contribute strong ideas to brainstorming sessions where senior staff are also present. But the guest list for a meeting where decisions are made typically includes more senior people and omits most junior staff.
In constructing a guest list, carefully think about whose input is needed, trying not to leave anyone out. Let the purpose of the meeting, not office politics, dictate who should be there.
In a similar vein, try to keep the guest list to a manageable number. Don't include people for the sake of including them. Only those who will advance the agenda need attend.
Consider your guests' time when inviting them to a meeting. If some people only need to attend part of the meeting, schedule a break to give them an opportunity to make an unobtrusive exit.
Politics and etiquette sometimes are at odds in guest lists for meetings. If you are not invited to a meeting you think you should attend, it probably won't serve you or the meeting well to make your way surreptitiously onto the guest list unless your omission was an oversight. Often it's impossible to tell, so it's best just to sit it out.
If you are systematically excluded from meetings you think you should attend, it could be a signal that your career inside the organization is at a dead end. On the other hand, if you are being invited to more and more meetings, it can be a sign of growth. As inconvenient as it can be to have much of your time taken up in meetings, it can signal that your contributions are vital.