Firstly, any people who you wish to admit to a Glow Group must be Glow users and you must be an adminstrator of that group.
You need to know the establisment they belong to. Add them as a member of the Glow Group in the usual way. Then let them know the url (address) of the Glow Group. As members of the group, when they click on the url link they will be taken to the Group.
However they will not see the group listed in their own "My Glow Groups" so if they will be visiting the group more than once it is best that they add the url link in a convenient web part in their "My Glow" group.