This is an interesting question. Some people just start proposal on Area51 thinking or hoping that interested people will find the proposal and join/follow it and eventually proposal will get success but actually things are opposite. Before starting a proposal, you should have sufficient mass committed to participate and contribute to the proposal. only then there are chances for a proposal to pass the definition phase.
Area51 FAQ states:
How do I start a new site?
... All we ask is that you have an enthusiastic, committed group of expert users who check in regularly, asking and answering questions.
Seven days of inactivity will lead to closure and removal of the proposal in the definition phase. So, without a solid group ready to participate in Definition phase, proposal is likely to be inviable. That's why Stack Exchange do ask to link organization/group ready to support the proposal:
Now coming to the point of question, the way/mode of gathering mass may depend upon the type of community you're going to build e.g whether it's a technological site or cultural site or a site belongs to arts, business etc.
You need to think for gathering and inviting people three times 1. Before making proposal 2. After making the proposal when it is in definition phase 3. When proposal is ready in beta phase
Before starting proposal on Area51:
In this stage you're supposed to have a group of people who gather and meet or talks through social network frequently. Formal discussion, introduction and invitation through mail to people or organization may not work here.
- It is expected that you have a solid group of people who are in your touch may be whom you know personally or talks daily or at least once in 3 to 4 days. They may be your close friends or family members or colleagues.
- Build a group on social networks (may be Telegram) and do daily conversations regarding the topics of proposal you're going to make.
- Invite interested people to your group and introduce them how are you all are going to start a proposal.
- If you have a fair connection to particular organization working on field relevant to topic of your proposal, approach it
- Once road-map is ready and clear to all people in the group, decide and start a proposal. Inform all the members how to join and post example questions.
During the definition phase:
- Bring your proposal to more and more people, make them join your proposal and your group for discussions; you can also think of creating chat-room on Stack Exchange if it works.
- Help others with joining the proposal and understanding how Area51 staging work, more precisely the definition phase.
- After your proposal starts looking clear and fine, start approaching organizations and institutions relevant to the domain of your proposal; introduce them about Stack Exchange, your proposal and Area51 process and request them to join or inform/make people belonging to that organization join the proposal
- Also try your hands in promoting your proposal to those Stack Exchange communities which might be interested in your proposal.
- Find groups on social network when you can advertise your proposal.
- Remind all members to vote on example questions occasionally.
I don't know much how commitment phase goes but when you succeed and pass through definition phase and likely also the commitment phase, start thinking on promoting private beta and public beta.
Keeping enthusiasm and interest among members of your proposal or community is essential to keep your proposal active. Once existing members will start enjoying it, they'll bring others to your proposal.