It's hard for me to judge shapeshifters because I can't see what forms can do. Do forms provide much tactical variety? Are there any first shapefocuses to avoid if I want to be useful to groups?

The most important roles of a shapeshifter in the group are damage dealer and tank. Defense can tank to some extent with it's second tier, well with it's third tier. Utility forms can tank to some extent with their third tier and a bit with their second tier(in optimal conditions). Utility can tank to well with it's fourth tier and defense can tank extremely well with it's fourth tier. Offense doesn't tank, but it deals out nice damage. Water forms are more powerful than the other tiers, but they are limited to water rooms(seas, rivers, underwater, swamp, ponds etc.) after the first form. The early forms of utility have neat perks(such as forage or hide) that can make the early forms useful even as hero. Utility is a good choise of focus for an explorer.

Basically, you should avoid water and air as first foci, but you may choose either of them as your minor focus if you so wish. Utility major water minor could work well for a Herald explorer, allowing you to explore both on land and under water.

Transmuter's role in the group is group utility, meaning that he increases the performance of others by hasting them, slowing and softening the mob, etc. It is generally harder for a muter to kill mobs alone than for a shifter to do so.

Herald will welcome goodies and neutrals, but evils could have some problems getting in.