NOTE – This post applies to Exchange Distribution Groups and Mail-Enabled Security Groups
To add an additional owner to a group that you own
After logging in to the Outlook Web App (OWA), select Settings 
> Options > groups > distribution groups I own
Double click on the group to manage and click ownership
In the Owners section, click 
When the Owners page loads, click in the 
Search people field and type the first and last name of the new owner and click on the
When the new owner appears in the list, click the next to their name and click 
Click save
To add members to a group that you own
After logging in to OWA, select Settings > Options > groups > distribution groups I own
Double click on the group to manage and click membership
In the Members section, click
When the Members page loads, click in the Search people field and type the first and last name of the new member and click on the
When the new member appears in the list, click the next to their name and click
Click save
To remove members from a group that you own
After logging in to OWA, select Settings > Options > groups > distribution groups I own
Double click on the group to manage and click membership
In the Members section, select the member to remove and click 
Click save
To modify membership approval settings for a group that you own
After logging in to OWA, select Settings > Options > groups > distribution groups I own
Double click on the group to manage and click membership approval
In the Choose whether owner approval is required to join the group section, select the appropriate option
In the Choose whether the group is open to leave section, select the appropriate option
Click save
To restrict who may send to a group that you own
After logging in to OWA, select Settings > Options > groups > distribution groups I own
Double click on the group to manage and click delivery management
In the If you want to restrict who can send messages to the group…section, click
When the Allowed senders page loads, click in the Search people field and type the first and last name of the allowed sender and click on the
When the allowed sender appears in the list, click the next to their name and click
Click save
To remove someone’s ability to send to a group that you own
After logging in to OWA, select Settings > Options > groups > distribution groups I own
Double click on the group to manage and click delivery management
In the If you want to restrict who can send messages to the group…section, select the sender to remove and click
Click save