Yes, a Windows account will have all of the permissions that are assigned directly to the login as well as any permissions that are assigned to any groups the account is a member of.
Permissions are cumulative, but DENYs will override. It would be difficult or impossible to accommodate some security needs if they were not.
See Effective SQL Server permissions when user is in several AD groups. The same principals apply to permissions assigned to a user and to a group they are a member of.
In general, DBAs that have been around a while will advise you to never start giving permissions to individual user accounts, as you can then end up with a long list of logins, and when employees leave you have to go clean them up. The preferred way is to only assign permissions to groups, and then when an employee leaves, or is hired, the DBA doesn't have to do anything as the request for access is granted by whoever manages the AD groups.