As there are 8 of you I would book in advance, but no need until a few weeks before.
There are 4 buses a day (7.00 - 13.00 - 16.30 - 20.30). Like Yiassas says: With 8 people it's best to book in advance. I'm not sure but I think it's possible from 1 month (or 4 weeks) before the actual date of travel.
You can book online through the website of the bus company: www.ktel-lefkadas.gr/…
You can only book max. 6 tickets at the same time
Book return tickets if you know your departure date,it is cheaper.
the intercity bus station in athens that serves this route is in kifissos area ..it is called kifissos station (ktel is the name of intercity buses in greece)...the station is rather grim but the buses are nice and punctual.
the current timetables are here
get to the bus station before the departure to get a ticket..i would get a taxi to get to the bus station,taxis are cheap here..if you download and use the BEAT app that we locals use, you will have a good service..it is regulated taxi drivers with a star rated system..Edited: 4:18 am, July 19, 2018
p.s..lefkada has an airport,so you could definitely fly there from athens airport..i do not know when you are going but if you find a good deal it can be really cheap,plus you will get to avoid the hustle of the bus,which will take the whole day...the actual flight is a few minutes..check the skyscanner site for flight tickets
This is inside the Kifissos busstation: https://flic.kr/p/rKRUkY
If you take a taxi to there from the city center or the Express bus X93 from the airport, they will drop you off near the grey kiosk you see on the right. The buses to Lefkada are just behing that kiosk. The office to buy tickets is in front of the bus.
Just to add, if you decide to fly, the name of the airport for Lefkada is Preveza/Aktio -- you can search by either of those names and you should find it. Some websites call it Preveza/Lefkada, but some don't, so if you just type in Lefkada you might not find it.
From Preveza/Aktio airport to Lefkada town is 25 minutes by bus or taxi.
Are there any other companies except ktel. Wanted to compare the prices.
No, there are no other bus companies.