Adding people to a board
To add members to a board, select "Invite" from the board menu. Search for a user by name or enter an email address to invite them to the board. Click their name to add them to the board. Depending on the board settings, you may need to be an admin to invite someone to the board.
You can add members to boards even if they are not already using Trello. Just enter the person's email address and click "Send Invitation." The user's permission level will be considered " Virtual" until they join Trello, but you can still add the user to cards.
Add other Trello members or invite new users to your board and collaborate in style!
Adding someone as an admin
You can make someone an admin for a board at the same time that you add them as a member. When you hover over a user's name on the "Invite" tab, you'll see an arrow on the right.
Clicking on the arrow will bring up the up their permissions settings. If the board is part of a Business Class team, you'll also have the option to invite a Guest as an Observer.
Select "Admin" from this menu and they will be added to the board as an admin.
Inviting people by using a shareable link
You can also invite others to your boards by sharing a special link with them. Read more about that here
Allow Team Members to Join
If you would like any of your team's members to be able to join the board and collaborate, you can select "Allow Team Members to Join" from "More"->" Settings" in the board menu. When you select this option a checkmark will appear next to "Allow Team Members to Join" and your team's members can add themselves to the board.
You can further control board permissions in the "Settings" option in the board menu (click "More" to expand the menu options). Under "Commenting Permissions," you can select who is allowed to comment on the board: no one, just board members, board members and observers, all team members (whether or not they're board members), or any Trello user.
Select who is allowed to comment on your board.
Using the "Invitations Permissions" menu, you can select who's allowed to invite new members to your board. If "Admins" is selected, only board administrators can invite new members. If "All Members" is selected, any board member can invite a new member to your board.
Troubleshooting Board Invitations
The user see a banner that says "You'll need to create an account for Trello to join the board" even though they're logged into an account registered to that email.
To fix this problem, the admin on the board in question will need to remove the invited Virtual User account from board and invite the user again using the same email address. That should link up with the right account now and grant the user access to the board as expected!