Use of 'join us'

[MountainGoat]

Do I have to use 'join us':
with the infinitive (e.g. join us to follow the trails),
with a preposition and gerund (e.g. join us in following the trails)
or with the gerund only (e.g. join us following the trails)?
Maybe every form is correct?

[queenbu]

In my opinion, only 'join us in....' is right.