You'll also be able to gage which agents check their email frequently and respond rapidly. Be specific about who you are, the benefits you perceive for membership, whom else you're inviting, the rules and the cost if any. Do continue to collect agent email addresses. Pick up one of the listing agent's cards when you preview or show a home. Jot it down from the MLS data or graphics sheet where available.
Many title companies collect lists of agent email addresses as a courtesy.
If not, suggest it. Do limit group email to text only. Even the fastest computers noticeably thud along when downloading photo attachments. Do encourage members to share the successes the network has brought them. It goes a long way to promote goodwill and continued participation among other members.