The steps below will help you remove and add users to all Microsoft groups you are currently the owner of. 

How to add users

1. Sign into your Outlook account on the Web version. 
2. Go over to the side toolbar and click on the People tab. (Figure 1)
   Figure 1:
   
3. Expand Groups, and then click on Owner (FIgure 2)
   Figure 2:
   
   
4. You will see the list of groups that you are the owner of. Select the group you are wanting to add a user to.
5. Click on the Members tab, and then click on Add members. (Figure 3)
   Figure 3:
   
   
6. Underneath Add members start entering in names or email address of users you want to add to the group. (Figure 4)
7. Once you have found a user, click on their name and their name will display on a list below. 
8. Make sure to have them marked as "Members". (Figure 4)
9. Once you have added all the members, go ahead and click on Add. (Figure 4)
   Figure 4:
   
   
10. You will receive a confirmation message stating that " # members successfully added ". You may then click on Close

The members should then receive a notice that they were added to the group. The notice should look like the screenshot below.

How to remove users

Follow steps 1-5 from above but do not click on Add members.

1. Go down the member list of who you want to remove and click on the "X" that is on the right
2. Click Yes

It may take a few seconds, but the member you removed will disappear from the list.