o.k....first you must join the tribe...then under the main photo you'll see invite friends to this tribe icon

also some new tribe.com subscribers don't know how to invite friends to there-own tribe
well go to you tribe and under the main photo of that tribe you'll see invite friends to this tribe

also a secret trick to inviteing non-friends to your tribe or any of the tribes you are a member of:

step1.....go to that persons profile
step 2...under that persons main photo you'll see and icon titled...more options...click that
step 3...you'll now see an icon titled invite this person to a tribe...click that

the rest is self explainitory