@Nerzarn What everyone has stated are all good advice.
So the best place to get members, is surprisingly in game. Play with randoms, if you see someone, nearby have a quick chat. It usually helps to look at players with lower levels, as sometimes they are newer players or solo players who are leveling alt characters. There's a bit of these out there. You will also run into people who are 1 man alliances, always chat with them if they are interested. I also posted a thread here https://forum.pso2.com/topic/2620/ship-1-check-out-shadow-cross-ngs-approved-update-8-21-20-full-open-recruitment-looking-for-active-members-pc-worldwide-steam-ship-transfer-veterans-new-players-returning?_=1598592321143 . I don't like bumping but once every week or other week I will post an update so people are aware. Because of the large player base and how many people have been dropping out of the game, I have hit 100 max capacity 3 times and dropped back to a better number. Still It's best to hit 100 for all the alliance points. I post on reddit and I also appointed a few officers who have earned it to help do exactly what I do. One even posted on the facebook pso page.
As for management, it is important you understand this 1 thing; as it holds true for any management/supervising title. Do you want to be the "Boss" or a "Leader". If you want to be a boss, then a team game like this may be the wrong way to go. A Leader is a person doing just as much as the members, if not most certainly more. I don't have to tell you, as with you asking it is clear that you care for your alliance and want to improve it what ever way you can. Having a website, I stopped paying for the site and just kept the domain and moved over to Discord as it is free and a lot easier to use. You don't need a website to get the most out of it, Discord is more than enough. I will post after this some of the channels I use to make it easier for my members to stay active. With more people in a discord, more people share bonds and strengthen your alliance. You won't want to leave if you are playing with friends over just some randoms in an alliance.
It is important to appreciate your members. Some members will work hard with you. Make sure you are paying attention to those online, helping others, posting, or donating to the alliance vault. I know 5 people in my alliance that just give away 10mil items like nothing because they want to be helpful or love everyone. Obviously you know there are people who would take advantage of them so you have to moderate to not let anyone be taken advantage of as well as maintaining a positive atmosphere where everyone feels welcomed and appreciated. A good way of doing this is with events. Be creative, there are no limits. I will give you the few that I have done, I usually try to do 1 a month, with group play over the weekends and such. Group Alliance photo (1st one done for promoting the alliance), 2nd one was a create an alliance flag symbol art to be displayed on the alliance quarters (next to your main one, as you have 3 monitors), Best Personal Quarters, another one was capture a funny fail, last one was Upload a funny moment video. I have 2 planned for October, one is best cosplay/costume and Best haunted house personal quarters theme. As for prizes, be creative. I buy $5 xbox gift cards to give away and I also try and buy small items/units/weapons and give out to winners. Prizes vary and sometimes others want to host and they provide prizes.
Let everyone in the alliance know if they see someone on or if during UQ to party up. This also helps with AP points for you which then you can spend on them. Keeping the tree boost upgraded and healthy is very important. I also promoted 2 other events. A open house, where most of the members where promoting on high volume blocks to come check out alliance/PQ for consideration, where we would talk to them about it they where interested in joining. Members where in alliance quarters giving out small fun scratch item prizes. It went well. Another very important thing is to push Discord. I had everyone who joined discord gets a small gift item and promoted to member. I felt it was best to demote every member to provisional. Those who join I know are active because I can monitor to some degree activity. Those who join you can have them promoted to member so you know they are active. As for those who don't or can't use Discord. Understand that not everyone will want to or like Discord. That is fine but you have to have equal ground. Have them send you a check in active message monthly. If they don't message them. If still nothing, you can assume inactive. For the most part be responsible and understanding. Life happens, if you know the person you have best judgment on call, but if they won't be able to for whatever reason let it be known that a simple hey I might be gone for a bit please don't kick or something is appreciated. There is no real way to monitor activity, You also do not want people taking advantage of others. While everyone is working to get AP done weekly they aren't pulling their weight and getting rewarded for it while others are working hard. It is important you are on top of this by using those methods. You can tell by usually Sunday, You have 3500 or so AP available per character X the amount of members should get you about this much weekly. Use those status comments to your advantage. Link the discord channel ,and have it as daily open message, with some other news to keep everyone on topic. I like to post any campaign news on discords general chat.
I know I posted a lot, but hopefully this helps you and some other players. Maybe it sparks some ideas. I wish tbh, I didn't have to use Discord and that everything was right there already. I have played many games with this Clan, It is important to love your alliance, them members, and whatever game you are playing together. Everyone has different time zones, life etc. Do not expect the same of every member, but at least try to aim for the minimum. All members should be doing AP missions for that weekly cap. After that they don't have to worry about it. You can always request screenshot evidence but don't be that guy. It is at the end of the day just a game. Make it enjoyable but manage it the best you can and as you want it. I hope that helped.