Ask the person what they need and how they want to be helped. They have lived in their own body for their whole life and have thought about this more than you have. Grabbing somebody, moving their wheelchair without asking, or taking an arm without warning is at best frightening and at worst injures them.
Who is missed in practice. People with limited mobility. People who are deaf or hard of hearing — who will not hear an alarm or a shouted instruction. People who are blind or partially sighted — in an unfamiliar route, in smoke, with the lights off. People with cognitive impairment or dementia, for whom a sudden change and a crowd are the emergency. People dependent on equipment: oxygen, a ventilator, dialysis, an insulin pump. People who do not speak the local language. Small children. People who will not leave an animal.
Communication.
- Face them, speak normally, do not shout, and keep your face visible — many people lip-read partially.
- Say what is happening and what you are going to do before you do it.
- Write it down or type it if speech is not working. A phone screen is a perfectly good way to say "fire, we are leaving now, this way".
- Use one clear instruction at a time.
Moving.
- A guide dog or assistance dog goes with the person. Always. It is not a pet and it is not negotiable.
- Mobility equipment goes with the person where it can. Somebody separated from their wheelchair at an assembly point is stranded there.
- Lifts are usually unusable in a fire. Know where the refuge points and evacuation chairs are before the day, and know who is trained to use them.
- Do not lift somebody without help and without asking how.
Medication and documents. Where there is a minute to spare: medication, the list of it, glasses, hearing aids, charger, identity documents. Where there is not, the person leaves and the rest is replaceable.
Never leave somebody alone intending to come back. Tell the emergency services exactly where anybody is who could not be moved — floor, room number, how many — and do not go back in yourself.
Plan for it in advance. Most of the above cannot be improvised in darkness and smoke. Any building where somebody needs help to leave needs a personal plan written for them and known to the people on that floor.
Source. World Health Organization, Disability and health fact sheet, https://www.who.int/news-room/fact-sheets/detail/disability-and-health ; WHO emergencies, https://www.who.int/emergencies
What this is not. Reading, not training, and not a fire-safety assessment. The evacuation plan of the building you are in governs.