There are even non-root apps that allow this (PDANet comes to mind.) There is a root app that works, but if you have xtrROM it should be on your phone already. I have actually never used it myself - I just don't need it - but I am 100% certain that there are people on this thread who have used it, and probably continue to.
There is no official tethering plan available for the Eris from Verizon - you cannot order it as an option, and they will not offer it to you if you have an Eris. There are Android phones that have tethering options, but not the Eris. Because the tethering is just carrying traffic on standard ports that can come from your phone, it would be very difficult for Verizon to detect that you are tethering with the account (except perhaps just by the amount of bandwidth consumed.)
You just have to realize before you do it that you are technically breaking the terms of the data plan service contract and Verizon can, I believe, actually disconnect your account and charge you an ETF. I can't remember if the contract says that they have to warn your first, but I doubt it/ The likelihood of that is very, very, very, very remote, I think - I have no read of a single person who was caught tethering illegally - but you should know that before you start.