If you only want to teleport people who are in a certain group, you can use the new scoreboard command to create a 'group' of people (using teams), and then teleport only that team with this command (targets nearest player in the group specified)
/tp @p[team=<internal group name>,<other params here>] <x> <y> <z>
To set up your groups, you can create a group like this:
/scoreboard teams add <internal group name> <display name>
And then add people to this group like this:
/scoreboard teams join <internal group name> <user name>
The advantage of this system is also that you can use it actually to separate groups in the actual game. The scoreboard command has a feature that means that the display name of a team will be displayed before the actual username, appearing like this in the chat:
[<display name>] <user name>: ...
More info on this system can be found in the wiki page linked above.
Changing the @p in the first command above to a player name will stop it from working, because parameters that are inside the [ and ] will only work for @p, @a, and @r. So you cannot substitute the first command above to /tp <player name>[r=2] <x> <y> <z>. It just won't work.
If you really want to use the r parameter on a single person, try using the above method that uses teams, but only put one person in the team. Here is a quick way to do this. Type these into the command console:
/scoreboard teams add <internal player name> <external player name>
/scoreboard teams join <internal player name> <player name>
And then in the command block, to teleport that person if they are in a radius of 2, you could use:
/tp @p[team=<internal player name>,r=2] <x> <y> <z>
Alternatively, you could just specify the name of the player in the selector. Following the previous example, if you want to teleport a certain person if they are in a radius of 2, you could use:
/tp @p[name=<player name>,r=2] <x> <y> <z>
Best Answer
You just have to add
.0to turn it into an exact number:teleports you exactly to 100.0 100.0 100.0 without centering you in the block.
Btw, this is not a behaviour of the relative teleport, so it wouldn't change anything in your example command. I assume your example command is in a command block, in that case it teleports you relative to the command block bottom center, so you would have to subtract 0.5 from the x and z coordinate to end up on a block corner.