As an event or group admin, there are abilities for you to individually add a person one by one or in bulk.
For an event: You will be "manually registering" a user and be given options to "select" an existing user to be registered, be added as a new user or you have the option to upload a flat file (excel, specifically a "csv" file) of those that you want to attend the event.
Note: If you want to provide further administrative capabilities to these users, you may do so by navigating to "Users" in the event's "Dashboard". There you can select a specific individual and grant them more abilities through the actions dropdown. If the individual doesn't exist yet, you can "Add User" and input manually their contact info and decide what roles to give them.
For a group: You will be "adding" users. This will give you the ability to manually enter in each of your members or users. In the case you want to bulk add them all in, you can do so via the excel ("csv") Upload.
For further assistance in regards to migration, onboarding your users, community partners, legacy data or existing data. Please feel free to contact our support team @ firstname.lastname@example.org or the "Support" button in the far right.