Teleporting a player from the overworld to the end

This is trivial in Minecraft 1.13:

/execute in the_end run tp <player> <location>

1.13+:

The introduction of the new execute in has made this task pretty easy:

/execute in the_end run tp <player> <location>

See: https://minecraft.gamepedia.com/Commands/execute#in

Before 1.13:

Not really. At least not directly. (something like /tp @p 0 50 0 dimension:end doesn't exist)

The only way would be to teleport them into an end-portal block (which you can create on any fixed position you'd like) and then to wherever you want (it takes some time to get to the end, so don't teleport them immediately).

You can spawn an end portal (like the one in the stronghold) one block above you:

 /setblock ~ ~1 ~ minecraft:end_portal