Hi!
I have around 25+ Members and a few of them fall to inactivity all of sudden(around 5)!
The thing that I do is first pointing them on our rules of inactivity!
If they doesn't react, I kick them and send a mail over our site writing why they were kicked and that they are welcome to join back but only if they keep a eye on our rules next time. Everyone should get a second chance.
Except if they were guildies with a bad behaviour or similar.
Though I must say that all the guildies I had (except of 2-3 jumpers!) were great people, nice, friendly and respectfull as I also have in our membership applications as a MUST.
My first tip to you would be to exactly point the recruits what kind of gameplay your guild is doing to see if they fit in your guild in the first place!
I had to decline to 3 people pointing them out that we are a PvE/PvP Guild that does both equal and wouldn't fit to their PvP needs.
This prevents them and us from dissappointment and leaves everyone a bit more happy(helps when it comes to the motivation).
The best thing you can do is: make Guild Events, Meetings, Training!
Give your guildies something to do TOGETHER so they can know each other better. You will see that the next time they will be online they will talk more with each other(even those that didn't before!).
Make them help each other or try to get them together in one of your missions/quests asking for "help" even if you don't need some.
Go in the Team Arena with some of your guildies or just say in the chat
"Hey Guys! Anyone wants to do some Arena?".
There are a lot of possibilities to keep your guild active as also to show your guildies that your guild is special and a place they like to stay.
There are always some that will leave for different reasons though.....
Sometimes you can prevent this and sometimes you can't!
Don't be disheartened because of this, just keep going on.
I have a Supergroup in CoH for over a year now and the only reason it starts to get more inactive then before (we were one of the most active supergroups on our server

) is because the people starts to get bored/vacations and other things.
Though we still are all in the activity limit so it is still alive and filled with great members I love to be with everytime I go there.