There are two ways to invite people to a Braid group. If you have already created the group, go into the group and tap the group name in the top title area. In Group Settings, in the members area, you should see "+ Invite Members". When you tap on that, you will be able to invite people to your group.
If you are creating a new group, once you select the type of group you will be prompted to add members to the group. You can select who you'd like to invite that way.