You have the ability to do many different bulk actions to apply to clients. These bulk actions can be done on the client's tab.
Step 1 - Select the clients you wish to apply the bulk action. In this case, we are adding them to the community.
Step 2 - In the Bulk Action bar, select Assign to a Community..
Step 3 - Select the Community or Communities you would like to add the client(s) to.
From this page, you can also add more clients to the list or you can remove clients here if you missed one.
Select Continue under the list of clients to add to the community.