Add photos and videos to an existing album by opening the album in edit mode: click 'edit'. Then click 'add' and choose 'photos and videos'.
Using the old editor? Then there's a bar on the left of your screen with elements you can add to your album. Choose 'Photos and videos' to add new photos or videos.
Which files?
Photos: JPG/JPEG, PNG or BMP of at least 400 x 400 pixels. HEIC, RAW and pdf don't work. Videos: up to 1 GB per file. With the Basic account, a video can be up to 5 minutes long. With Premium or Pro, there's no limit on length.
High quality
Photos and videos in albums on the free Basic account are stored in high quality: photos up to 16 megapixels, video up to 1080p.
Several photographers?
MyAlbum also has group albums. Click the at the top right of the album and choose 'contribute...' to add your photos and videos. You need an account for this. The owner of the album gets a request to approve your photos and videos.