When playing Realm of the Mad God and wandering around sometimes another player appear suddenly near me, like he/she has just teleported there. Is it possible or are there other explanations?
You can teleport to anyone at any time by typing "/teleport [playername]" in the chat, by clicking on the person's name and clicking "teleport", or by clicking a yellow dot in the minimap.
Teleporting does have a cooldown, so you can't spam it nonstop, although it's not very long (only a few seconds).
To answer your question, you can teleport in 4 ways. You can teleport by typing /teleport [playername] in the chat, clicking a yellow dot on the map and then clicking teleport, clicking on a nearby person in the bottom right corner then teleport, or you can teleport to your guild members in blue or the people you locked in green by clicking them in the bottom right and click teleport. You can't teleport in oryx's castle unless you have a trickster with a teleporting ability. Tricksters have a ability if you don't know that they can teleport. The cooldown of teleporting is 10 seconds.