You must be a Super Administrator, Community Administrator or a Member Administrator to do this.
Navigate to your community by clicking hovering over the community icon (the globe) and clicking on your community's name under owned communities.
If you want to exchange forms (eg. Purchase orders or invoices) with your community members, we suggest you setup your form settings and member types first. Find out how to do that here.
On the left navigation, click community settings > member settings > members and click invite.
Enter the details and select the member type.
Choose to add a message to the email Colladium sends and click send invitation.
You can also choose to invite by CSV upload. This allows you to select a CSV of your contacts, map the columns and send all the invitations at once.