I have helped get a couple masters rowing groups going after "lulls" and helped rebuild. The key is to allow anyone and everyone, and not be all "elitist" and "picky" about who you let on the team.
You are all adults, and all have "real" lives now. Work, family, other commitments. Rowing is now just something you do for fun, it is not your life. Sometimes I have coached "masters" level rowers that treat rowing as their life way into their 30s, but they are sad people that nobody really likes to row with.
Start off by having practices on weeknights and maybe early one morning on a weekend. Most people with kids will have weekend mornings off and the kids will be home watching saturday morning cartoons. Weeknights after work is a great way to forget about work problems. More than two nights a week and husbands/wives will get mad that they are not spending enough time with the family.
As long as your keep the team open and allow anyone with any previous rowing experience, you can start to BUILD the team. Face it, you need bodies to make a 4 and an 8.
At the beginning you will need to have a coach that can bring together all the types of rowing in the boat. Some guys might come from clubs, others from "snobby" schools like Yale, others might just have learned to row after college. Wherever they come from they NEED to get together and change to match the coach's vision of technique. If you have, say, a harvard snob, calling all the shots, because he went to Harvard, then, make him the coach. But really some of the best coaches are the ones from out of nowhere. Take it from me, you can come from a State school that won nothing significant and turn a team into a nationally recognized program.
Outside of the regular water practices it would be a good idea to encourage erg workouts, but keep in mind Masters level guys might not have that much time.
When it comes time for competition, it is totally ok to have mixed up boats with "all" level of guys and then break out into A, B and C boats in the 4s. This way the faster and better guys have a chance at medals, but then EVERYONE feels like a member of the team. I think that the slower guys know they are slower, no need to keep them from practicing in the same boat as the "fast" guys. The goal is to get everyone rowing with correct form and technique on the water and then just before races really work to boat the lineups you are racing.
As people start to hear and learn about the program, and interest grows, you will get more people on the team. Because you kept the door open to anyone, they will see it is a low pressure club and FUN. Trust me, you dont want the type of guys that are "oh.. they allow anyone? not just guys from harvard and Yale? oh, well then you are not serious, I dont want to join" Those are the worsr type of rowers for an adult team and what I call "boat cancer".
Lastly, make sure you specify WHO is in charge. It should be the coach, not the captain. The captain and the coxswains need to do the Coach's bidding. Not the other way around. I have coached teams where the Captain and Cox made the decisions on who rowed in what boat, and it destroyed an amazing and fun team . The coach should not play favorites, and everyone should get equal treatment and priority for boats at practice. Seat racing and erg results should be fair and upfront for big races like Head of the Charles etc.