My Question is below.The osu (outcasts) of Mbanta were permitted to do all of the following except a. join the missionaries in their new religion...

The osu are of course the outcasts of the Igbo society and they
are thrown out for a variety of reasons.  Some of them were unfortunate enough to bear twins and
are then considered unclean along with their twin children and are banished.  For other reasons
they have been thrown out and were expected to allow their hair and beards to grow so that they
could be differentiated from the members of the tribe that are in good
standing.

So they are allowed to join the Christians because the
Igbo generally ignore the church and think that nothing will come of it.  They are encouraged by
the missionaries to shave their beards and cut their hair and join the
church.