Yammer allows you to invite members who are outside of your organisation, so that they can participate in groups alongside you and your colleagues, within the same interface.
For external parties to use Yammer with you, you simply need to add their email address to the group's member list and they will receive an invitation. If they do not already have a Microsoft account associated with their email address, they may need to fill out a quick form to register, after which they will be provided the access that you have given them.
There are two primary levels to external collaboration within Yammer:
Within your primary Yammer network, when creating groups, you can specify that they are "External" when creating them. These groups work in the same manner as regular groups, except that you are able to add external email addresses to the member list.
If you require a greater level of collaboration than an individual group, perhaps if you are inviting a large amount of users in to participate in numerous groups, you can create an External Network. These are, in principle, a fresh instance of Yammer, which is owned and secured to your organisation, connected to your primary network. Those who you invite in to an external network can only see the contents within there, and have no access to your primary corporate network in Yammer.