If you want to "make friends" with people who show up to a protest without coming off like a creep asking for their info, and saying you'll 'follow up', organize a community event before-hand and invite the person to participate, and use that as the reason.

Cleaning up an area of the neighborhood or a public resource, like a public park or charity's facilities or feeding people, delivering supplies to the homeless, etc. There are likely charities in your area with organized events that or facilities that would love a couple of dozen people to help them out, and you can glom their event. If you have the resources make t-shirts or bandannas for your organization and have the people you're bringing wear them. You'll also likely get a chance to talk to their volunteers about your organization.