If you really want to build an active guild, you need to give your members a good reason to both join your guild and be active.
A good thing to do would be set out clear objectives of your guild and stick to them.
My current guild is a "have fun" type guild, where pretty much everyone only logs on for events or updates and is very attractive for plenty of old time players that simply don't have anything else to do on a day-to-day basis. The drawback is there are usually a max of 4 members online when there isn't an event. I know quite a few other guilds that are in a similar status as my guild.
If you want constantly active members, you need to look towards newer players that haven't yet finished all the missions. Devoting time towards assisting players completing missions would definitely be an attractive feature to get them to join and stay in your guild.