View with Page display won't show menu tab for an alias
Create your views as follows:

Challenges - 

path: node/%/challenges

tab: menu tab with title Challenges

filter criteria: type = group

Contextual filter: nid, and in section WHEN THE FILTER VALUE IS IN THE URL OR A DEFAULT IS PROVIDED specify validation criteria: Validator Content, Content types group

Members- 

path: node/%/members

tab: menu tab with title Members

filter criteria: you con't need any as you are displaying users (you have a view of users)

relationship: add a relationship to the node table such as: User: Content authored. This will bring in the related nodes.

Contextual filter: nid, in the section rlationships nodes must be selected. Specify validation criteria as Validator Content, Content types group

And these two tabs will now show up only for node type group. If you need it on other content types as well then adjust your views filter criteria and contextual filter validation as required.