You can view and edit your contacts within Mailspring! Just choose Window > Contacts from the app's main menu. Mailspring supports Google Contacts and CardDAV, an open standard used by iCloud, FastMail, and others. We do not currently allow you to manage contacts from Outlook, Office365 of Yahoo accounts.
To create a new contact, select the "All Contacts" group of one of your connected accounts from the sidebar. The "Add Contact" button in the toolbar will become enabled once you've selected one of your accounts. Then just click Add Contact, fill in your contact's information, and press Save Changes in the bottom right.
Creating and Managing Contact Groups
From the left sidebar of the contacts window, you'll see a list of your connected accounts and the contact groups you've created. To create a new group, hover over the account's label and click the "+" icon that appears.
Add members to the group by dragging and dropping them from the account's "All Contacts" view or from another group.
Remove members from the group by selecting one while viewing the group and choosing "Remove from Group" from the toolbar in the top right.
To remove or rename a contact group, right click it or double click it in the left sidebar where groups are shown.
When you're composing new emails, type the name of a Contact Group into the recipients box and press Enter to automatically add all of the members of the group (that have email addresses) to your message.