Yes, a linkshell is effectively a guild, although their are differences from other games that you will have to look into. There are some guide around, I know there is one on FF Vault: http://ffvault.ign.com/ It explains how to form/invite/join a linkshell, look in the General Guides section. Be aware it will cost you about 8000 Gil to start one up.
Back in the days before luclin, my EQ guild raided alot, so many times we would have 50+ members in a single zone, there were times when we had another guild, or made an alliance with another guild and we could have raids of as many as 80 people, this was probably to take on a god or something, usually by the time we got to it many would leave leaving around 40 to finish up. There was also an inflow of other members loggin on or mercenaries joining up, so during a raid, esp a full plane raid, I would guess as many as 100+ people might fight for us. The ideal number would be 50 in a raid, anymore and you ended up with management issues, anyless and things could slow down alot. I think at it's peak my guild had like 118 members, but we were highly selective, not only did you have to be in the top 10% of levels (55+), but you had to be a decent player, and honourable person, that made recruiting tough, but the members we got were top-notch people.
Edited, Sat Jan 24 19:01:30 2004 by Ralluwen