Adding and removing recipients from distribution lists is the responsibility of the distribution list owner. This owner is typically set at the creation of the distribution list. If you are not the distribution list owner, then you will not be able to add or remove recipients. If you are the owner, here's the steps you'll take to edit the list.
1) Navigate to outlook.office.com and log in. This will require authentication via the Microsoft MFA app on your mobile device.
2) Click the gear icon in the upper left corner of the screen, and then click the Mail hyperlink located towards the bottom of the Settings toolbar.
3) Click the triangle next to General to expand that section, and then select Distribution groups.
4) Give the site a few minutes to populate this screen. Once populated, you'll see a list on the left of Distribution groups that you belong to, and on the right a list of Distribution groups that you own. Select the list that you wish to edit, and then click the Pencil icon in the upper left.
4) You will be presented with a pop up window to edit the list. Click Membership on the navigation pane on the left, and click "+" to add members. To remove members, select the user you want to remove, and click the "-" button. Once you're done adding and removing users, click the Save button in the lower right corner of the window. This will close the window, and you're all set!