Personal & low-volume vs impersonal & high-volume
Recruiting is primarily about communicating the importance of allyship, awareness, and action. Communication isn't just the content of the message, but how it's delivered and who is delivering it.
When it comes to communication there is a tradeoff between quality and quantity. You can easily send an email to hundreds of people within a few seconds, but odds are low that a big percentage will read it. Most group leaders default to sending only mass emails, but you miss out on many people who simply don't read most emails.
You can also easily have a short personal conversation with a few people, but doing so with hundreds of people would take an inordinate amount of time. And unless you're pretty extroverted and don't mind starting conversations with acquaintances, you'll probably only speak to those you already know. There's also a lot of middle ground between these two extremes.
Create a communication strategy
Use a combination of different communication strategies.
Whether you're trying to get membership signups, newsletter subscribers, or event attendees, the following advice holds true. First, use personal communication for those most likely to sign up for membership or participate in an event. Second, remember the Marketing Rule of 7, which basically says that people need to “hear” a message at least 7 times before they’ll do something. You don't want to overdo it and piss people off, but in today's busy world, you'll achieve best results if you communicate out on different channels. For example, (1) send an email to your group email list, (2) create a calendar event and share with everyone on your email list, (3) get other student leaders to announce on their email lists, (4) post an event on social media, (5) make a classroom announcement, (6) send personal texts to others, and (7) have personal conversations with some folks.
The most effective way
Make a spreadsheet with everyone's names and email addresses. Mark down the levels at which you think people might want to participate (something simple: enthusiastic, receptive, skeptical, antagonistic). On your first communication round about membership and the weekly newsletter, ask everyone. Don't assume either way: you will may be surprised about who cares and who doesn't. If people aren't interested, don't bother them again.
Reach out one-to-one to as many people as possible. Best case: talk to them in-person. Second best: send an individualized text (sent only to them with their name). Last: individualized email to them (sent only to them with their name).
You might be thinking, that's a lot of work. Here's the good news. You don't have to reach out to each person yourself, especially given you probably don't know everyone well enough to reach out one-to-one. Round up a team (i.e. allyship group leadership and volunteer Partner Group members). Have team members assign themselves to talk to particular people on the spreadsheet you've created. That way, nobody has to reach out to more than 10 or 15 individuals, which only takes a few minutes.
Haas signed up 97% of the men in their class for a male allyship newsletter using this method. They contacted 100% of them and did it in two weeks. They also signed up one third of their male classmates as club members within a week. You can use this method for membership sign-ups, email signups, event participation, pretty much anything.
Communicating and recruiting for events
Follow the Marketing Rule of 7 guidelines in the section above. Get a few dedicated allyship group members and send individual texts to the people who you think would appreciate it. Most people, even if they want to come to your event, may easily miss a single email in the morass of email everyone gets daily. Sending an individual communication is harder to miss and so they're more likely to make the event.
Additionally, most people who want to come probably will be busy and can't make it anyway. It's just the nature of life today. Generally speaking, people appreciate receiving a few texts a month reminding them of an event that they probably want to go to if they have the time.