There are numerous ways to add a contact to a mailing list.

In Workspace > Email Marketing > Contacts, the + LIST and + RECIPIENT menus can be used.

In Workspace > CRM > Constituents, a contact profile record can be edited and the Email Lists field updated.

Some DonorView pages allow a constituent to be added to a mailing list when the page is submitted.

And various grids include Actions > Create Email List which allows a new list to be created, or an existing list to be updated.

Using Actions > Create Email Lists is the only way to include additional mail merge fields (details from the module) to be used in a campaign template.