On the same trains yes almost definitely. Trains with mandatory seat reservations (mostly high-speed trains) would be the biggest issue for seating together. On all other trains no problem.

You can sometimes state seat preference but not through eurail.com. You'll have to go through the railway companies which actually will save you money anyway...

The only exception would be trains where the passholder quota might be sold out (a rare occurence) : Thalys, Eurostar, TGVs to Belgium.

However if your friend is doing the same trip, why wouldn't they buy a pass ? That would solve a lot of hassle... Make some calculations depending on your route and let us know for further questions.

Which countries will you be travelling in?

Austria, Italy, Spain, Greece, Denmark, France, maybe Greenland and Hungary

Italy and France might be a bit tricky to reserve seats next to each other. In Spain you’ll have to reserve your seats at the ticket office anyway, so you can ask to be seated next to each other.

For the rest seat reservations are optional, so you can buy multiple in once, being seated next to each other.

But if your friend is following you, it’s definetly cheaper to buy a pass for her too (unless she only joins for a shorter stretch)